Fitch’s paradox of knowability explores a puzzle in epistemic logic—a branch of logic concerned with knowledge—by showing a surprising consequence of the “knowability thesis,” which claims that every truth can, in principle, be known.
The paradox points out that if all truths are knowable, then it actually follows that every truth is already known, a much stronger claim called the “omniscience principle.”
The reasoning is that if there’s a truth that nobody knows—an “unknown truth”—then, assuming knowability, it should be possible to know that “this truth is unknown.” But as soon as it becomes known that a truth is unknown, it can no longer be unknown, creating a contradiction. Therefore, under the knowability thesis, unknown truths cannot exist, so every truth must be known.
This challenges the views of verificationists and anti-realists, who believe truth is closely tied to what can be verified or known, since they find the idea of omniscience implausible.
The logic behind the paradox minimally assumes standard principles about knowledge (“if something is known, it’s true,” and “if each part of something is known, the whole is known”) and possibility (“every truth is possibly knowable”), but these generate the contradiction.
The paradox can be generalized to other concepts like belief or causation, showing its broader impact on how we think about knowledge and possibility.
Online Resources
- Fitch’s paradox of knowability / Wikipedia
- Fitch’s Paradox of Knowability / SEP
Please Note: This is my personal summary of the topic, shared both for my own records and in the hope it may be helpful to you. AI was used in parts to assist with the process.