This article is an interpretation of Zbigniew Herbert's poem 'Thomas'. It is generally assumed that the writing of the poem was inspired by Caravaggio's painting 'The Incredulity of St Thomas'. However, as the author of this article claims, an equally important inspiration for that poem is to be found in Denise Levertov's 'St Thomas Didymus', a poem translated into Polish by Czeslaw Milosz. In the context of the latter source, Herbert's verse can be read as a poetic vision of the doubting Thomas and an affirmation of man's attempts to question and verify religious truths. For Herbert such questioning is of great significance, both on the level of poetic art and as a source of poetic inspiration.