Agnosticism (Greek: α- a-, without + γνώσις gnōsis, knowledge; after Gnosticism) is the philosophical view that the truth value of certain claims — particularly metaphysical claims regarding theology, afterlife or the existence of deities, spiritual beings, or even ultimate reality — is unknown or, depending on the form of agnosticism, inherently impossible to prove or disprove and hence unknowable. It is not a religious declaration in itself and the terms are not mutually exclusive.

Demographic research services normally list agnostics in the same category as atheists and/or non-religious people, using agnostic in the sense of noncommittal. However, this can be misleading given the existence of agnostic theists, who identify themselves as both agnostics in the original sense and followers of a particular religion.

Many philosophers and thinkers have written about agnosticism, including Thomas Henry Huxley, Albert Einstein, Robert G. Ingersoll, and Bertrand Russell. Religious scholars who wrote about agnosticism are Peter Kreeft, Blaise Pascal and Cardinal Joseph Ratzinger, later elected as Pope Benedict XVI.