My Adaptation of Nick Thomas-Brown’s Argument for Idealism

P1) If, at a given time, a true proposition about every Object, at all times, is known relative to one or more Subjects, then every Object, at all times, is of such a nature as to be knowable relative to one or more Subjects, at a given time.

P2) If every Object, at all times, is of such a nature as to be knowable relative to one or more Subjects, at a given time, then no Object that exists, has existed, or will exist, is of such a nature as to be unknowable relative to one or more Subjects, at all times.

“Let us call something, X, “relatively” unknown when some but not all people fail to know propositional truths about X. Furthermore, we will call X “absolutely” unknown if, and only if, nobody knows any propositional truths concerning X. Now, it is clear that relative unknowns exist. Most people know nothing about the Cynic philosopher Hipparchia, but those who study ancient Greek philosophy surely know of her. Thus, Hipparchia is unknown relative to certain people at certain times. Suppose someone, A, does not know about Hipparchia at a time T. Given this, Hipparchia is unknown relative to A at T. It would, then, be a contradiction for A to know of Hipparchia at T. Hence, once it is admitted that Hipparchia is not known by A at T, it follows that she cannot be known by A at T. So, we can reasonably support the following epistemic theorem: whatever is unknown relative to someone at a certain time, is unknowable relative to her at that time. But, suppose that instead of being relatively unknown, Hipparchia is absolutely unknown. In other words, let us consider that Hipparchia is not known by anyone at any time. We might observe that this can be restated as the following: Hipparchia is unknown relative to any person at any given time. When combined with our discovered theorem, we can infer from this that Hipparchia is unknowable relative to any person at any time. Thus, we have reached a second epistemic theorem: whatever is absolutely unknown, is unknowable by anyone whatsoever.” (Thomas-Brown, Idealism and the Known Unknown, 3) 

C1) Therefore, if at a given time, a true proposition about every Object, at all times, is known relative to one or more Subjects, then no Object that exists, has existed, or will exist, is of such a nature as to be unknowable relative to one or more Subjects, at all times. [From P1 and P2]

P3) If no Object that exists, has existed, or will exist, is of such a nature as to be unknowable relative to one or more Subjects, at all times, then every Object that exists, has existed, or will exist, depends upon its being known relative to one or more Subjects, at a given time.

“Granted our two theorems, we can move to demonstrate that nothing absolutely unknown ever exists. We know as certainly as we can that any given entity is either square or not square, for this is an instance of the law of excluded middle. In grasping this universal truth, we know a truth concerning all things past, present, or future. If an absolute unknown existed, we would have some knowledge of it by way of such universal truths. But, it is a contradiction for an absolute unknown to be known by someone or other. Hence, if we are to have knowledge of universal truths at any time, there can never exist absolute unknowns. Yet, we do, of course, possess such knowledge; and, thus, no absolute unknowns exist, past, present, or future. Whatever exists or comes to exist is, therefore, known by someone or other at some time or other. In other words, to be is to be known and existence implies knowledge. Once we understand that something’s existence entails its being known, we can further see that its existence depends on its being known. However, the presence of knowledge presupposes that of a knower. Hence, the existence of a given object depends on there being an intelligent self or selves.” (Thomas-Brown, Idealism and the Known Unknown, 3-4)

C2) Therefore, if at a given time, a true proposition about every Object, at all times, is known relative to one or more Subjects, then every Object that exists, has existed, or will exist, depends upon its being known relative to one or more Subjects, at a given time. [From C1 and P3]

P4) At a given time, a true proposition about every Object, at all times, is known relative to one or more Subjects.

C3) Therefore, every Object that exists, has existed, or will exist, depends upon its being known relative to one or more Subjects, at a given time. [From C2 and P4]

All credit for the original version of this particular argument goes to Nick Thomas-Brown. Thomas-Brown published the argument in a 2019 paper titled “Idealism and the Known Unknown.”