The extended substance to Descartes is broadly the material, whereas the thinking or res cogitans is the thinking substance. How to bridge the immaterial mentality of thinking as a causal force in the material presented a problem for him and was difficult to bridge in this conceptualization of the full human: the mind body problem. The split subject in Lacanian psychoanalysis refers to in part the "mirror stage" of psychological development, where the undeveloped infant first sees it's own image in a mirror and begins to recognize it's own incomplete self image (only seeing of itself without the mirror its legs, arms, perceiving its clumsy motor skills, etc) compared to the ideal subject reflected in the perceived-to-be complete "other" in the mirror, which is not first recognized as itself but an ideal form which seems to be superior and complete in comparison. This is furthered by the imposition of language or in Lacanian terms "castration" which is essentially the imposition of a defining, limiting, prohibitive symbolic structure by way of language which creates a separating or splitting of the self independent of the flesh and blood, now extant in the symbolic and signified sense. There can be no unification of the split self, which is the underlying source of the neurotic or psychotic, with the former responding to castration with repression and the latter responding with a complete rejection of the prohibitions circumscribed therein. This is a vulgar understanding but how I interpret it.