This paper begins by summarizing Jean-Claude Milner’s equation (from his A Search for Clarity: Science and Philosophy in Lacan’s Oeuvre) between subject of science and subject of unconscious as developed by Lacan, with particular emphasis on Descartes’ cogito as a predecessor of both. It then focuses on Milner’s peculiar remark, in which he parallels Descartes’ “escape” from non-substantial cogito (devoid of any substance) to res cogitans (a substantial thing) with Lacan’s anecdote of “three prisoners” from his early essay “Logical Time”. By taking into account Lacan’s insight that what triggers the “escape” of three prisoners is precisely the “ontological form of anxiety”, the paper argues that it is the same form, the same horror vacui that causes Descartes’ immediate move from cogito (as a materialisation of void) to res cogitans. It is on this basis that Lacan later (in Seminars XIV The Logic of Fantasy and XVII The Other Side of Psychoanalysis) introduces the disjunction between being and thought (“I think where I am not, and I am where I don’t think”).