Hobbes called thinking "mental discourse" to distinguish it from speaking or "verbal discourse". This may seem a fairly obvious distinction but it raises the question "Where does the material, the stuff we think about, come from?". There have been two alternative views to this. One notion that dates all the way back to Pythagoras but was adopted in variations by Plato and Descartes, is that we are born with the contents of the mind in place and that in learning, we are recalling contents of the mind already in place. This notion was countered by Locke, who claimed that all the material of thinking derives from experience. The mind is a blank sheet upon which experience writes. Kant tried to resolve these oppositions by assuming that all minds processed information in the same way - we are born with a set of concepts and categories in common which determine how we think rather than what we think. Various techniques have been suggested to try and identify the nature of the contents of the human mind, or as it has been called, its "life world" (Husserl). It is only when ideas are expressed, made public through behaviour, language or culture that they can be examined or studied and each of these have been studied with a view to understanding the deeper structure of the human mind. (Freud, Wittgenstein, Lévi-Strauss)