That Which Knows All Things and Is Known by None: An Argument Against the Identity Theory of Mind

 Abstract

“That which knows all things and is known by none is the subject.”¹

“He sees, but he can’t be seen, he hears, but he can’t be heard; he perceives, but he can’t be perceived.”²

This essay presents an argument against the Identity Theory of Mind”³ that does not depend upon the apparent epistemic gap between physical and phenomenal truths, the existence of qualia, or the conceivability of philosophical zombies. The argument begins by establishing three principles: the Principle of Perceptual Determinacy, the Principle of Perceptual Self-Exclusion, and the Principle of Perceptual Non-Loopability—each identifying a necessary condition underlying the existence, or occurrence, of perceptual states. Next, a contradiction is shown to follow from the three principles and the Identity Theory being jointly true. Since the three principles can be demonstrated a priori, it follows that the Identity Theory is false. Potential objections to the argument are then raised and addressed. 

The Principle of Perceptual Determinacy

“When you perceive you perceive something, and…what you do perceive cannot be the same as the perception of it.”⁴

The world unfolds itself for us conscious beings through communions of perceivings and perceiveds: perceptual states and their percepta. These distinguishable factors march two abreast, side-by-side, hand-in-hand, as mutually-implying correlates within a subject’s experience. For a subject to “perceive” a tree is for him to “have” a perceptual state that is a “perception of a tree;”⁵ thus, if a subject has a perceptual state, P, and P is a “perception of a tree,” then P’s perceptum is that tree which the subject perceives—that particular “somewhat” which P is a “perception of.”⁶

Whenever a subject perceives, the perceptum of his perceptual state is an existing, or occurring, determinate “somewhat.” Moreover, this perceived “somewhat” appears to him, the perceiving subject. If the perceptum of a subject’s perceptual state was not an existing, or occurring, determinate “somewhat,” then the perceptual state in question (i.e., the “perceiving”) would be a “perception of nothing,” and this is absurd. For, no existing, or occurring, perceptual state is a “perception of nothing.” Furthermore, if the perceptum of a subject’s perceptual state was not itself determinate, then the perceptum would be incapable of appearing to and for the perceiving subject—and this, of course, is absurd. Indeed, unlike an existing, or occurring, determinate “somewhat,” an indeterminate would lack any definite characteristics of its own, and, as such, would not admit to being perceived in any particular way, let alone a variety of ways.⁷ However, a perceptual state is determinate only insofar as it terminates in an existing, or occurring, determinate perceptum. Indeed, a perceptual state is what it is (i.e., is determinate and individuated) in virtue of possessing characteristics that are fixed or determined by what the perceptual state is a “perception of.” If this were not so, then it should be possible to render a perceptual state determinable independent of the perceptual state’s objective reference (i.e., without any reference to what it is a “perception of”); however, such a task is impossible not only in practice but in principle. Since no perceptual state can either exist, or occur, unless it is determinate, and a perceptual state is determinate only insofar as its perceptum either exists, or occurs, and is determinate, we are driven to the following result: The Principle of Perceptual Determinacy. According to this principle, for any perceptual state, P, P can neither exist, nor occur, unless P’s perceptum, O, either exists, or occurs, and is determinate. 

The Principle of Perceptual Self-Exclusion

“That the subject should become object for itself is the most monstrous contradiction ever thought of...”⁸

Using the Principle of Perceptual Determinacy, it can be demonstrated that no existing, or occurring, perceptual state either has itself as its own perceptum, or is identical to its own perceptum. In other words, for any existing, or occurring, perceptual state, P, P is not a “perception of P.” I call this the Principle of Perceptual Self-Exclusion. The following argument illustrates how the Principle of Perceptual Self-Exclusion follows a priori from the Principle of Perceptual Determinacy:

If a perceptual state, A, was a “perception of A,” then A could neither exist, nor occur, nor be determinate, unless it was the perceptum of an existing, or occurring, perceptual state, B, that was a “perception of a perception of A.” But B could neither exist, nor occur, nor be determinate, unless it was the perceptum of an existing, or occurring, perceptual state, C, that was a “perception of a perception of a perception of A.” But C could neither exist, nor occur, nor be determinate, unless it was the perceptum of an existing, or occurring, perceptual state, D, that was a “perception of a perception of a perception of A.” But D could neither exist, nor occur, nor be determinate, unless it was the perceptum of an existing, or occurring, perceptual state, E, that was a “perception of a perception of a perception of a perception of A.” But E could neither exist, nor occur, nor be determinate, unless it was the perceptum of an existing, or occurring, perceptual state, F, that was a “perception of a perception of a perception of a perception of a perception of A.” But F could neither exist, nor occur, nor be determinate, unless it was the perceptum of an existing, or occurring, perceptual state, G, that was a “perception of a perception of a perception of a perception of a perception of a perception of A.” But G could neither exist, nor occur, nor be determinate, unless it was the perceptum of an existing, or occurring, perceptual state, H, that was a “perception of a perception of a perception of a perception of a perception of a perception of a perception of A.” And so on, ad infinitum. We find that neither A nor any of the other aforementioned perceptual states, could either exist, or occur, or be determinate, unless we had reached the end of a vicious regress of perceptual states—and this, of course, is impossible. Therefore, no existing, or occurring, perceptual state either has itself as its own perceptum, or is identical to its own perceptum. Therefore, the Principle of Perceptual Self-Exclusion is true.

The Principle of Perceptual Non-Loopability

“Knowledge that turns inward upon itself creates a current so hot that it burns out the fuse.”⁹

In a similar vein to the Principle of Perceptual Determinacy and the Principle of Perceptual Self-Exclusion, I will introduce what I call the Principle of Perceptual Non-Loopability. This principle maintains that no n-number (n ≥ 2) of existing, or occurring, perceptual states form a series of perceptual states and percepta wherein the first state is a “perception of the second,” the second is a “perception of the third,” the third is a “perception of the fourth,” and so on—the last perceptual state in the series being a “perception of the first.” For clarity, I have provided a list of eight examples of perceptual “loops.”

A Digonal Perceptual “Loop”

Any situation wherein two perceptual states are such that the first is a “perception of the second,” and the second is a “perception of the first.”

A Trigonal Perceptual “Loop”

Any situation wherein three perceptual states are such that the first is a “perception of the second,” the second is a “perception of the third,” and the third is a “perception of the first.”

A Tetragonal Perceptual “Loop”

Any situation wherein four perceptual states are such that the first is a “perception of the second,” the second is a “perception of the third,” the third is a “perception of the fourth,” and the fourth is a “perception of the first.”

A Pentagonal Perceptual “Loop”

Any situation wherein five perceptual states are such that the first is a “perception of the second,” the second is a “perception of the third,” the third is a “perception of the fourth,” the fourth is a “perception of the fifth,” and the fifth is a “perception of the first.”

A Hexagonal Perceptual “Loop”

Any situation wherein six perceptual states are such that the first is a “perception of the second,” the second is a “perception of the third,” the third is a “perception of the fourth,” the fourth is a “perception of the fifth,” the fifth is a “perception of the sixth,” and the sixth is a “perception of the first.” 

A Heptagonal Perceptual “Loop”

Any situation wherein seven perceptual states are such that the first is a “perception of the second,” the second is a “perception of the third,” the third is a “perception of the fourth,” the fourth is a “perception of the fifth,” the fifth is a “perception of the sixth,” the sixth is a “perception of the seventh,” and the seventh is a “perception of the first.”

An Octagonal Perceptual “Loop”

Any situation wherein eight perceptual states are such that the first is a “perception of the second,” the second is a “perception of the third,” the third is a “perception of the fourth,” the fourth is a “perception of the fifth,” the fifth is a “perception of the sixth,” the sixth is a “perception of the seventh,” the seventh is a “perception of the eighth,” and the eighth is a “perception of the first.”

And so on, ad indefinitum.

If the Principle of Perceptual Non-Loopability is true, it would follow that no n-number (n ≥ 2) of existing, or occurring, perceptual states form a series of perceptual states and percepta wherein the first perceptual state in the series is a “perception of the second,” the second is a “perception of the third,” the third is a “perception of the fourth,” and so on—the last perceptual state in the series being a “perception of the first.” Like the Principle of Perceptual Self-Exclusion, the Principle of Perceptual Non-Loopability follows a priori from the Principle of Perceptual Determinacy. This can be demonstrated by means of the following argument:

If two perceptual states, A and B, were such that A was a “perception of B,” and B was a “perception of A,” then A and B could neither exist, nor occur, nor be determinate, unless A was the perceptum of an existing, or occurring, perceptual state, C, that was a “perception of a perception of B,” and B was the perceptum of an existing, or occurring, perceptual state, D, that was a “perception of a perception of A.” However, C and D could neither exist, nor occur, nor be determinate, unless C was the perceptum of an existing, or occurring, perceptual state, E, that was a “perception of a perception of a perception of B,” and D was the perceptum of an existing, or occurring, perceptual state, F, that was a “perception of a perception of a perception of A.” However, E and F could neither exist, nor occur, nor be determinate, unless E was the perceptum of an existing, or occurring, perceptual state, G, that was a “perception of a perception of a perception of a perception of B,” and F was the perceptum of an existing, or occurring, perceptual state, H, that was a “perception of a perception of perception of a perception of A.” And so on, ad infinitum. We find that neither A, nor B, nor any of the aforementioned perceptual states, could either exist, or occur, or be determinate, unless we had reached the end of a vicious regress of perceptual states—and this, of course, is impossible. Therefore, no two existing, or occurring, perceptual states are such that the first is a “perception of the second,” and the second is a “perception of the first.” Therefore, the Principle of Perceptual Non-Loopability is true.

One might object by saying that the above argument only demonstrates that a digonal perceptual “loop” can neither exist, nor occur. However, this objection rests on an overlooking the fact that the same conclusion follows if we were to substitute the digonal perceptual “loop” for a trigonal, tetragonal, pentagonal, hexagonal, heptagonal, octagonal (and so on, ad indefinitum) perceptual “loop.” Thus, we can affirm that the Principle of Perceptual Non-Loopability has been proved.

An Argument Against the Identity Theory of Mind

Let’s imagine two neighbors: John and Thomas. If John were to have a perceptual state, A, and A was a “perception of Thomas’s perceptual state, B,” and if Thomas’s perceptual state, B, was a “perception of John’s perceptual state, A,” then John’s and Thomas’s perceptual states would form a digonal perceptual “loop.” However, since we have proved the Principle of Perceptual Non-Loopability, it follows that such a digonal perceptual “loop” can neither exist, nor occur. Therefore, if John were to have an existing, or occurring, perceptual state, A, and A was a “perception of Thomas’s existing, or occurring, perceptual state, B,” then Thomas’s existing, or occurring, perceptual state, B, cannot—on pain of contradiction—be a “perception of John’s existing, or occurring, perceptual state, A.”

According to the Identity Theory, all of John’s perceptual states are identical to some of John’s brain states, and all of Thomas’s perceptual states are identical to some of Thomas’s brain states. In other words, if John were to have an existing, or occurring, perceptual state, A, then A would be identical to John’s existing, or occurring, brain state, θ, and if Thomas were to have an existing, or occurring, perceptual state B, then B would be identical to Thomas’s existing, or occurring, brain state, ψ. Furthermore, since the Principle of Perceptual Non-Loopability is true, we arrive at the following result: 

If the Identity Theory is true, then if John has an existing, or occurring, perceptual state, A, and A is a “perception of Thomas’s existing, or occurring, brain state, ψ,” then Thomas’s existing, or occurring, brain state, ψ, cannot—on pain of contradiction—be a “perception of John’s existing, or occurring, brain state, θ.”

Now, there is nothing absurd or contradictory in the idea that John can perceive (i) any of his own existing, or occurring, brain states, and (ii) any existing, or occurring, state of Thomas’s brain whilst Thomas perceives any existing, or occurring, state of John’s brain. Likewise, there is nothing unintelligible or self-contradictory in the idea that Thomas can perceive (i) any of his own existing, or occurring, brain states, and (ii) any existing, or occurring, state of John’s brain whilst John perceives any existing, or occurring, state of Thomas’s brain. Indeed, we must admit that it is possible—in principle—for John to perceive (i) any of his own existing, or occurring, brain states, and (ii) any existing, or occurring, state of Thomas’s brain whilst Thomas perceives any existing, or occurring, state of John’s brain. Again, we must also admit that it is possible—in principle—for Thomas to perceive (i) any of his own existing, or occurring, brain states, and (ii) any existing, or occurring, state of John’s brain whilst John perceives any existing, or occurring, state of Thomas’s brain. However, if the Identity Theory is true, then it would be a contradiction for John to perceive a certain existing, or occurring, state, ψ, of Thomas’s brain whilst Thomas is perceiving a certain existing, or occurring, state, θ, of John’s brain. Indeed, such a state of affairs would be impossible—in principle. Therefore, since it is possible—in principle—for John to perceive any existing, or occurring, state of Thomas’s brain whilst Thomas perceives any existing, or occurring, state of John’s brain, it follows that the Identity Theory is false. We may put the general argument in the following form:

P1)  If the Identity Theory is true, then John cannot—in principle—perceive any existing, or occurring, state of Thomas’s brain whilst Thomas perceives any existing, or occurring, state of John’s brain.

P2)  John can—in principle—perceive any existing, or occurring, state of Thomas’s brain whilst Thomas perceives any existing, or occurring, state of John’s brain.

C1)  Therefore, the Identity Theory is false. [From P1 and P2]

We may transpose the above argument in a way that focuses solely on John. This variation of the argument relies solely on the Principle of Perceptual Self-Exclusion without relying on the Principle of Perceptual Non-Loopability. The Identity Theory, as we have seen, is committed to John’s existing, or occurring, perceptual state, P, being identical to one of John’s existing, or occurring, brain states (at the time of John’s having of the perceptual state, P). Since the Principle of Perceptual Self-Exclusion is true, it follows that whenever John has an existing, or occurring, perceptual state, P, P’s perceptum, O, cannot—in principle—be John’s existing, or occurring, perceptual state, P. However, it follows from this that if it is possible—in principle—for John to have an existing, or occurring, perceptual state, P, that is a “perception of” any one of his existing, or occurring, brain states (at the time of John’s having of the perceptual state, P), then John’s existing, or occurring, perceptual state, P, is not identical to any one of his existing, or occurring, brain states (at the time of John’s having of the perceptual state, P). But if John’s existing, or occurring, perceptual state, P, is not identical to any one of his existing, or occurring, brain states (at the time of John’s having of the perceptual state, P), it follows that the Identity Theory is false; for, such a theory is committed to John’s existing, or occurring, perceptual state, P, being identical to one of John’s existing, or occurring, brain states (at the time of John’s having of the perceptual state, P). Since it is possible—in principle—for John to have an existing, or occurring, perceptual state, P, that is a “perception of” any one of his existing, or occurring, brain states (at the time of John’s having of the perceptual state, P), it follows that the Identity Theory is false. We may standardize the argument as follows:

P1)  If the Identity Theory is true, then all of John’s existing, or occurring, perceptual states are identical to some of John’s existing, or occurring, brain states.

P2)  If all of John’s existing, or occurring, perceptual states are identical to some of John’s existing, or occurring, brain states, then some of John’s existing, or occurring, brain states cannot—in principle—be the percepta of John’s existing, or occurring, perceptual states. 

P3)  All of John’s existing, or occurring, brain states can—in principle—be the percepta of John’s existing, or occurring, perceptual states. 

C1)  Therefore, the Identity Theory is false. [From P1—P3]

The Identity Theorist cannot reject the argument’s first premise without giving up the Identity Theory, and the Identity Theorist cannot challenge the second premise without first refuting the Principle of Perceptual Self-Exclusion—something which I take to have been sufficiently proved. Furthermore, the Principle of Perceptual Self-Exclusion follows a priori from the Principle of Perceptual Determinacy—another principle which I take to have been sufficiently proved. Therefore, if the Identity Theorist is to reject the Principle of Perceptual Self-Exclusion, he must also reject the Principle of Perceptual Determinacy. With respect to the third premise, there is clearly nothing absurd or contradictory involved in the idea that John can—in principle—perceive any given existing, or occurring, state of his own brain whilst having an existing, or occurring, perceptual state. Indeed, if John’s optic nerves were lengthened and his eyes positioned correctly, he would certainly be able to perceive his own brain states (Not to mention the possibility of John using an autocerebroscope to perceive his brain states).¹⁰ Now, suppose the Identity Theorist were to argue that whenever a perceptual state (or a brain state) is perceived, it cannot be perceived in its entirety, but only partially, thus a perceptual state (or a brain state) having itself as its own perceptum would present no difficulty. Such an objection fails because it would lead to the following contradiction: a proper part of a whole (i.e., the perceived part of the whole perceptual state) would be both less than, and identical to, the whole (i.e., the whole perceptual state) of which it was a proper part.

Now, there is a third argument that we may employ against the Identity Theory that makes use of the Principle of Perceptual Self-Exclusion and the Principle of Perceptual Non-Loopability—both of which follow a priori from the Principle of Perceptual Determinacy. The argument takes the following form:

P1)  If the Identity Theory is true, then there are some brain states which are—in principle—imperceptible. 

P2)  There are no brain states which are—in principle—imperceptible. 

C1)  Therefore, the Identity Theory is false. [From P1 and P2]

As mentioned in the preceding paragraphs, there is nothing at all incoherent or contradictory in the idea that all existing, or occurring, brain states are—in principle—perceptible; indeed, it seems that it is not only possible in theory, but also—eventually—in practice.¹⁰ As the force of the argument lies in the first premise, I will concentrate my attention on defending it the most. The first premise is based on the fact that if the Identity Theory is true, then all existing, or occurring, perceptual states are identical to some existing, or occurring, brain states. Supposing that all existing, or occurring, perceptual states are identical to some existing, or occurring, brain states, it can be demonstrated that there are some existing, or occurring, brain states which are—in principle—imperceptible. The argument is as follows:

If an existing, or occurring, perceptual state, A, is a “perception of an existing, or occurring, perceptual state, B,” then A is neither the perceptum of B, nor the perceptum of A; because if A was the perceptum of either A or B, then either the Principle of Perceptual Self-Exclusion, or the Principle of Perceptual Non-Loopability would be violated. However, this entails that A is imperceptible—in principle—unless A was the perceptum of an existing, or occurring, perceptual state, C,—a perceptual state that was different from A and B.

Indeed, if A was the perceptum of A, then we would have an existing, or occurring, perceptual state that had itself as its own perceptum—a clear violation of the Principle of Perceptual Self-Exclusion. Furthermore, if A was the perceptum of B, then we would have a digonal perceptual “loop”—a clear violation of the Principle of Perceptual Non-Loopability. 

However, this entails that C is imperceptible—in principle—unless C was the perceptum of an existing, or occurring, perceptual state, D,—a perceptual state that was different from A, B, and C.

Indeed, if C was the perceptum of C, then we would have an existing, or occurring, perceptual state that had itself as its own perceptum—a clear violation of the Principle of Perceptual Self-Exclusion. Furthermore, if C was the perceptum of either A or B, then we would have either a digonal, or trigonal, perceptual “loop”—a clear violation of the Principle of Perceptual Non-Loopability. 

However, this entails that D is imperceptible—in principle—unless D was the perceptum of an existing, or occurring, perceptual state, E,—a perceptual state that was different from A, B, C, and D.

Indeed, if D was the perceptum of D, then we would have an existing or occurring, perceptual state that had itself as its own perceptum—a clear violation of the Principle of Perceptual Self-Exclusion. Furthermore, if D was the perceptum of either A, B, or C, then we would have either a digonal, trigonal, or tetragonal perceptual “loop”—a clear violation of the Principle of Perceptual Non-Loopability. 

However, this entails that E is imperceptible—in principle—unless E was the perceptum of an existing, or occurring, perceptual state, F,—a perceptual state that was different from A, B, C, D, and E.

Indeed, if E was the perceptum of E, then we would have an existing, or occurring, perceptual state that had itself its own perceptum—a clear violation of the Principle of Perceptual Self-Exclusion. Furthermore, if E was the perceptum of either A, B, C, or D, then we would have either a digonal, trigonal, tetragonal, or pentagonal perceptual “loop”—a clear violation of the Principle of Perceptual Non-Loopability. 

However, this entails that F is imperceptible—in principle—unless F was the perceptum of an existing, or occurring, perceptual state, G,—a perceptual state that was different from A, B, C, D, E, and F.

Indeed, if F was the perceptum of F, then we would have an existing, or occurring, perceptual state that had itself as its own perceptum—a clear violation of the Principle of Perceptual Self-Exclusion. Furthermore, if F was the perceptum of either A, B, C, D, or E, then we would have either a digonal, trigonal, tetragonal, pentagonal, or hexagonal perceptual “loop”—a clear violation of the Principle of Perceptual Non-Loopability.

And so on, ad infinitum.

We are forced into a vicious, infinite regress. Every new perceptual state being posited (as existing, or occurring) to account for another perceptual state’s imperceptibility would itself be imperceptible—in principle—unless an additional perceptual state was posited (as existing, or occurring); however, this additional perceptual state (posited as existing, or occurring) would itself be imperceptible—in principle—unless a further perceptual state was posited (as existing, or occurring), and so on, ad infinitum. Therefore, since the regress is vicious, it follows that there are some existing, or occurring, perceptual states which are—in principle—imperceptible. However, this means that if all existing, or occurring, perceptual states are identical to some existing, or occurring, brain states, then some existing, or occurring, brain states are—in principle—imperceptible. But, as we saw above, all existing, or occurring, brain states are—in principle—perceptible. Therefore, it follows that some existing, or occurring, perceptual states are not brain states. Since the truth of the Identity Theory rests upon all existing, or occurring, perceptual states being identical to some existing, or occurring, brain states, it follows that the Identity Theory is false.

 

I have provided a diagram illustrating the vicious regress and the two-horned dilemma which follows from it. Each of the circles represent a particular perceptual state (e.g., Perceptual State A, Perceptual State B, Perceptual State C, Perceptual State D, Perceptual State E, and Perceptual State F). If a circle is green, then said perceptual state is the perceptum of another perceptual state (i.e., It is perceived). If a circle is red, then said perceptual state is not the perceptum of another perceptual state (i.e., The perceptual state is imperceptible). A black arrow pointing from one perceptual state to another perceptual state signifies that the former perceptual state is a perception of the latter perceptual state (e.g., The black arrow pointing from Perceptual State A to Perceptual State B signifies that Perceptual State A is a perception of Perceptual State B). A red arrow pointing from one perceptual state to another perceptual state signifies that the former perceptual state is not, and cannot be, a perception of the latter perceptual state—on pain of violating the Principle of Perceptual Self-Exclusion (e.g., The red arrow pointing from Perceptual State B to Perceptual State A signifies that Perceptual State B is not, and cannot be, a perception of Perceptual State A—on pain of violating the Principle of Perceptual Self-Exclusion). Perceptual situations contained in red boxes violate either the Principle of Perceptual Self-Exclusion (e.g., Fig. 1) or the Principle of Perceptual Non-Loopability (e.g., Fig. 3, Fig. 5, and Fig. 7). Perceptual situations contained in green boxes neither violate the Principle of Perceptual Self-Exclusion, nor the Principle of Perceptual Non-Loopability; however, in such perceptual situations, there is always a remaining perceptual state—an imperceptible one—that is left unaccounted for (e.g., Fig. 2, Fig. 4, Fig. 6, and Fig. 8).

Addressing Two Objections

First Objection

“I wondered why one couldn’t perceive the contents of perception at different times. Perhaps when a tree is the perceptum of a perceptual state (brain state B), I could later have a new perceptual state to perceive brain state B. Then all the brain states would still be perceptible in principle.”

My concern with the proposed solution is that it faces the same dilemma that would arise if the perceptual states were simultaneous. Let me try to illustrate the issue:

If an existing, or occurring, perceptual state A was a “perception of a perceptual state, B,” and B had either existed, or occurred, prior to A, then A could have neither been the perceptum of B (since A hadn’t even existed yet to be the perceptum of B), nor could A have been the perceptum of A (without violating the Principle of Perceptual Self-Exclusion). However, it follows from this that A would be imperceptible—in principle—unless it was the perceptum of another existing, or occurring, perceptual state, C,—a perceptual state that was itself different from A and B. Now, it is clear that if C either existed, or occurred, later than A and B, then C could have neither been the perceptum of A, nor the perceptum of B (since C hadn’t even existed yet to be the perceptum of either A or B). However, if C was the perceptum of C, then the Principle of Perceptual Self-Exclusion would have been violated. Thus, C would be imperceptible—in principle—unless it was the perceptum of another existing, or occurring, perceptual state, D,—a perceptual state that was different from A, B, and C. And so on, ad infinitum.

Just as before, every new existing, or occurring, perceptual state needing to be posited to account for another perceptual state’s imperceptibility would itself be imperceptible—in principle—unless another existing, or occurring, perceptual state were posited; however, this existing, or occurring, perceptual state would itself be imperceptible—in principle—unless another existing, or occurring, perceptual state were posited, and so on, ad infinitum. In short, no matter how many perceptual states we posit, we will always get stuck with a “imperceptible” remainder left unaccounted for.

Second Objection

If I follow, the core point is that a perceptual state’s character is determined by its perceptum—the object of the perceptual state (q.v. The Principle of Perceptual Determinacy); and since a perceptual state’s character is “fixed” or rendered determinate by its perceptum, it follows that the perceptum of a perceptual state must have a character independent of that perceptual state. From this it seems to follow that (i) a perceptual state cannot be a “perception of itself” (q.v. The Principle of Perceptual Self-Exclusion), (ii) no two perceptual states can be of one another, and (iii) no n-number (n ≥ 2) of perceptual states can form a series of perceptual states and percepta wherein the first perceptual state in the series is a “perception of the second,” the second is a “perception of the third,” the third is a “perception of the fourth,” and so on—the last perceptual state in the series being a “perception of the first.”  (q.v. The Principle of Perceptual Non-Loopability). However, if the Identity Theory is true, and all perceptual states are identical to some brain states, then it is possible—in principle—for a perceptual state to be a “perception of itself”; indeed, it is possible—in principle—for someone to have a perceptual state of any of their brain states. For example, a person can perceive any of their brain states (a) in a mirror, (b) by having their optic nerves surgically extended—a medical operation which, if not capable of being performed with today’s technology, is by no means incapable of being accomplished in the near future, or even (c) by means of Feigl’s hypothetical autocerebroscope. Furthermore, if the Identity Theory is true, and all perceptual states are identical to some brain states, then it is possible—in principle—for two perceptual states to be of one another; indeed, it is possible—in principle—for two craniotomy patients to be positioned in such a way so that they perceive one another’s brain states simultaneously. Likewise, if the Identity Theory is true, then it is possible—in principle—for n-number (n ≥ 2) of perceptual states to form a series of perceptual states and percepta wherein the first perceptual state in the series is a “perception of the second,” the second is a “perception of the third,” the third is a “perception of the fourth,” and so on—the last perceptual state in the series being a “perception of the first.” Indeed, like before with two craniotomy patients, it is possible—in principle—for n-number (n ≥ 2) of craniotomy patients to be arranged in such a way that the first patient in the group perceives any one of the brain states of a second patient, the second perceives any one of the brain states of the third, the third perceives any one of the brain states of the fourth, and so on—the last patient in the group perceiving any one of the first patient’s brain states. Therefore, the Identity Theory is false. It seems that the best reply for an Identity Theorist would be to say that it is not true that a perceptual state’s character is determined by its perceptum. Rather, a perceptual state represents an object/character that may or may not exist. And so, there is no problem with a perceptual state being of itself, any more than there is a problem with other cases of reflective representation (e.g., the novel, Don Quixote, is, in part, about the novel, Don Quixote).”

There appear to be a couple of difficulties entailed by this suggested response. My first concern is that there seems to be a serious dilemma when it comes to the self-representation involved in a perceptual state having itself as its own perceptum. Coincidentally, A.E. Taylor’s criticism of Josiah Royce’s conception of the Absolute as a self-representing system parallels the issues involved in the actual implementation of a perceptual state that had itself as its own perceptum:

“Professor Royce’s own illustration of the map of England executed upon a portion of the surface of the country is really a typical instance of a self-contradictory purpose. He argues that such a map, to be theoretically perfect, must contain a reduced facsimile of itself as part of the country mapped, and this again another, and so on indefinitely. But the whole force of the reasoning depends on overlooking the distinction between the surface of England as it is before the map is made, and the surface of England as altered by the presence of the map. Prof. Royce assumes that you set out to represent in the map a state of things which can in fact have no existence until after the map is made. The previous existence of the map at a certain spot is falsely taken to be one of the conditions to which the map-maker is to conform in executing it. Every one of the supposed “maps within the map” will thus involve distortion and misrepresentation of the district it proposes to map. It is as if Hamlet had chosen “Hamlet” as the subject of the “play within the play.” The professor’s illustration thus does less than justice to his theory.”¹¹

My second concern is that the proposed objection (i.e., that no existing, or occurring, perceptual state has any of its characteristics determined by the characteristics of its perceptum) ends up vindicating the Principle of Perceptual Self-Exclusion—the very principle the objection sought to dissolve. The self-undermining character of the proposed objection can be made more explicit with the help from the following argument (an argument inspired by J.M.E. McTaggart’s conception of “Intrinsic Determination”¹² as a relation between characteristics):

Let X represent the characteristic of “having the characteristic of ‘neither existing, nor occurring, if perceptual state, P, neither exists, nor occurs’.”  

Let Y represent the characteristic of “neither existing, nor occurring, if perceptual state, P, neither exists, nor occurs.” 

Now, the occurrence of X as a characteristic in a given thing “intrinsically determines” the occurrence of Y as a characteristic in a given thing because the proposition “something has the characteristic X” implies the proposition “something has the characteristic Y.” As a matter of fact, the occurrence of X as a characteristic in a given thing determines the occurrence of Y as a characteristic in that very same thing which possesses X as a characteristic.  

Now, if an existing, or occurring, perceptual state, P, has none of its characteristics determined by any of the characteristics of its perceptum, then it immediately follows that no existing, or occurring, perceptual state, P, has itself as its own perceptum. This can be demonstrated in the following manner: 

If an existing, or occurring, perceptual state, P, had itself as its own perceptum, then P’s perceptum would have the characteristic of “having the characteristic of ‘neither existing, nor occurring, if P neither exists, nor occurs’.” However, the “having” of this characteristic by P’s perceptum would thereby determine the occurrence of the characteristic of “neither existing, nor occurring, if P neither exists, nor occurs,” in P’s perceptum. However, ex hypothesi, P’s perceptum is P itself. Thus, X being a characteristic of P’s perceptum would intrinsically determine Y as a characteristic of P. In other words, if no existing, or occurring, perceptual state, P, has any of its characteristics determined by the characteristics of its perceptum, it follows that no existing, or occurring, perceptual state, P, has itself as its own perceptum. And since the Principle of Perceptual Self-Exclusion holds that no existing, or occurring, perceptual state, P, has itself as its own perceptum, it follows that the Principle of Perceptual Self-Exclusion is true.

The proposed objection to our argument (i.e., that no existing or occurring perceptual state has any of its characteristics determined by the characteristics of its perceptum) has thus ended up vindicating the Principle of Perceptual Self-Exclusion.

Concluding Remarks

“I am not what I see, / And other than the things I touch.”⁹

If the preceding arguments are sound, then the Identity Theory of Mind has been refuted—not by an appeal to the apparent qualitative character of our subjective life, but through an exploration of the necessary conditions underlying the existence and occurrence of perceptual states and their percepta.

Footnotes

^[1] Arthur Schopenhauer, The World as Will and Idea, trans. Richard Burdon Haldane and John Kemp, vol. I, III vols. (London, England: Trübner & Co., 1883), 5.

^[2] Patrick Olivelle, ed., “Bṛhadāraṇyaka Upaniṣad,” in The Early Upaniṣads: Annotated Text and Translation, trans. Patrick Olivelle (New York, New York: Oxford University Press, 1998), 29–165, 3.7.23.

^[3] John Jamieson Carswell Smart, “The Mind/Brain Identity Theory,” in The Stanford Encyclopedia of Philosophy, ed. Edward Nouri Zalta and Uri Nodelman, Winter 2022 Edition (Stanford, CA: Metaphysics Research Lab, Stanford University, 2022).

^[4] Charlie Dunbar Broad, Perception, Physics, and Reality: An Enquiry Into the Information That Physical Science Can Supply about the Real (London, England: Cambridge University Press, 1914), 5.

^[5] The italicized “is” should be interpreted as the “is” of identity. Thus, for any perceptual state, P, P is identical to a “perception of a perceptum, O.”

^[6] Just as every perceptual state has a correlative perceptum, so too does every perceptum have a corresponding perceptual state. Thus, for any perceptum, O, there is a particular “somewhat” which is identical to the “perception of” that perceptum, O.

^[7] It is vital to insist upon two important points. Firstly, it is for a perceptual state’s perceptum needn’t be perceived accurately for it to be the perceptum of the aforementioned perception state. Suppose that (i) a perceptual state, A, is a “perception of a “somewhat” as having property f,” and (ii) there just so happens to be no existent, or occurrent, “somewhat,” that has property f. Although A’s perceptum is perceived as having properties that it does not, in fact, have (i.e., the property f), it does not follow that A’s perceptum (i.e., the particular “somewhat” which is erroneously perceived as having property f) neither exists, nor occurs, nor is determinate. On the contrary, just as perceptual state A exists, or occurs, and is determinate, A’s perceptum exists, or occurs, and is determinate—the perceptum is simply misperceived or erroneously perceived. Secondly, one may broaden the definition of “perceptum” so that it would be the following: “For any existent, or occurrent, perceptual state, P, a perceptum is whatever serves to render P existent (or occurrent) and determinate as “this” perceptual token rather than indeterminate nothing. In cases of veridical perception, the perceptum of P would be something in the percipient subject’s environment. In cases of illusion or hallucination, the role (of perceptum) is still filled (albeit not necessarily by a worldly object external to the subject’s body), but something must serve to determine P as of something. Moreover, a perceptual state is determinate if and only if it possesses its individuating character sufficient to (type-)identify it as “this” very perceptual (token).

^[8] Arthur Schopenhauer, Critical Debates: (1809 – 1818), ed. Arthur Hübscher, trans. Eric Francis John Payne, vol. II, IV vols., of Manuscript Remains (Oxford: Berg, 1988), 349.

^[9] Charles Henry Rieber, Footnotes to Formal Logic (Berkeley, California: University of California Press, 1918), 10.

^[10] Now, “perceptible-in-principle” and “imperceptible-in-principle,” like “knowable-in-principle,” are, in fact, genuine or “extensional” properties, rather than merely “intensional” properties. The fact that “perceptible-in-principle” and “imperceptible-in-principle” are extensional properties renders the preceding argument immune from accusations of committing a “masked-man” or “intensional” fallacy.

^[11] Alfred Edward Taylor, Elements of Metaphysics, 12th ed. (London, England: Methuen & Co. Ltd, 1961), 150.

^[12] Cf. John McTaggart Ellis McTaggart, The Nature of Existence, vol. I, II vols. (Cambridge, England: Cambridge University Press, 1921), 111.

^[13] Alfred Tennyson, In Memoriam (London, United Kingdom: Henry S. King & Co., 1876), sec. 45, lines 7–8.

Bibliography

Broad, Charlie Dunbar. Perception, Physics, and Reality: An Enquiry Into the Information that Physical Science Can Supply about the Real. London, England: Cambridge University Press, 1914.

McTaggart, John McTaggart Ellis. The Nature of Existence. Vol. I. II vols. Cambridge, England: Cambridge University Press, 1921.

Olivelle, Patrick, ed. “Bṛhadāraṇyaka Upaniṣad.” In The Early Upaniṣads: Annotated Text and Translation, 29–165. New York, New York: Oxford University Press, 1998.

Rieber, Charles Henry. Footnotes to Formal Logic. Berkeley, California: University of California Press, 1918.

Schopenhauer, Arthur. Critical Debates: (1809 – 1818). Edited by Arthur Hübscher. Translated by Eric Francis John Payne. Vol. II. IV vols. of Manuscript Remains. Oxford: Berg, 1988.

Schopenhauer, Arthur. The World as Will and Idea. Translated by Richard Burdon Haldane and John Kemp. Vol. I. III vols. London, England: Trübner & Co., 1883.

Smart, James Jamieson Carswell. “The Mind/Brain Identity Theory.” Edited by Uri Nodelman and Edward Nouri Zalta. Stanford Encyclopedia of Philosophy, Winter 2022. https://plato.stanford.edu/archives/win2022/entries/mind-identity/.

Taylor, Alfred Edward. Elements of Metaphysics. 12th ed. London, England: Methuen & Co. Ltd, 1961.

Tennyson, Alfred. In Memoriam. London, United Kingdom: Henry S. King & Co., 1876.