Stace's philosophy of mysticism grew out of his earlier empiricist epistemology, although this is something many critics of his position fail to appreciate. The concept of the 'given', commonly used in phenomenalism to understand the nature of experience, is crucial to both his earlier epistemology and his later analysis. For Stace it lies at the basis of our knowledge of the external world and of ourselves. The given has an important epistemological function because it possesses the properties of certainty (infallibility, incorrigibility, indubitability), and it provides the ultimate justification for all forms of human knowledge. Stace's Theory of Knowledge and Existence (1932) explains that knowledge arises from the process of interpretation of the given, although he writes that it is not easy to distinguish between the given and interpretation of it. For Overall, the 'pure experience' or 'sensation' he refers to in Mysticism and Philosophy (1960) is the same as the given that he had been writing about earlier.