Hans Vaihinger (/ˈvaɪhɪŋɡər/; German: [hans ˈfaɪɪŋɐ];[3] September 25, 1852 – December 18, 1933) was a German philosopher, best known as a Kant scholar and for his Die Philosophie des Als Ob (The Philosophy of 'As if'), published in 1911 although its statement of basic principles had been written more than thirty years earlier.

[2] Vaihinger acknowledged several precursors, especially Kant,[2]: vii–ix, xxiii–xlviii  and Hermann Lotze and wrote that he felt vindicated by Friedrich Albert Lange,[2]: XIII  but had been unaware of Jeremy Bentham's Theory of Fictions until, at the very end of his life, it was brought to his attention by his translator, C. K.

[2]: v–vi [6] In the preface to the English edition of his work, Vaihinger expressed his principle of fictionalism: "An idea whose theoretical untruth or incorrectness, and therewith its falsity, is admitted, is not for that reason practically valueless and useless; for such an idea, in spite of its theoretical nullity[,] may have great practical importance.

Vaihinger’s philosophy of 'as if' can be viewed as one of the central premises upon which George Kelly's personal construct psychology is based.

Kelly credited Vaihinger with influencing his theory, especially the idea that our constructions are better viewed as useful hypotheses rather than representations of objective reality.

However, the American journalist Mencken was scathing in his criticism of the book, which he dismissed as an unimportant "foot-note to all existing systems".

[12] After his death, and the intellectual sea change that followed the Second World War, Vaihinger's work received little attention from philosophers.

[12] Vaihinger's influence has since markedly increased, and the currently booming fictionalism movement in the philosophy of science takes his contributions as its main historical lead and inspiration.