unknowable

Unknowable is a term used in philosophy and everyday language to denote propositions, entities, or domains that lie beyond the reach of human knowledge or possible knowledge. It signals a limit to what can be known, either because of cognitive constraints, epistemic methods, or theoretical boundaries.

The idea is often divided into epistemic unknowability (we cannot know it with any possible evidence, given our methods) and ontological or absolute unknowability (no mind could ever know it, regardless of method).

In philosophy, Kant distinguishes phenomena (things as they appear to us) from noumena (things in themselves) which are, by definition, unknowable. In logic and mathematics, Gödel's incompleteness theorem shows that any sufficiently powerful formal system contains true statements that cannot be proven within the system, illustrating systematic limits to knowledge rather than a claim about reality itself.

In science, many limits of knowledge are framed as epistemic unknowability, such as the precise state of the early universe beyond the observable horizon, or the exact value of certain quantities at Planck scales. The quantum-mechanical uncertainty principle describes limits on simultaneous knowledge but does not declare those quantities unknowable in principle; interpretations vary. The unknowable is also a common motif in theology and literature, used to convey mystery or the limits of human comprehension.

Critics warn that talk of the unknowable can reflect current methodological constraints rather than metaphysical necessity. Proponents emphasize humility and the historical possibility that future theories could render some currently unknowable aspects knowable, while others may remain beyond epistemic reach.