‘Extimatrix’ reconsiders conventional assumptions separating human (consciousness, intentions, intelligence) from technical objects, images and things. Looking at philosophical and scientific versions of the ‘brain in a vat’ thought experiment, the essay re-examines the (untenable) division between inside and outside. The notion of extimatrix proposes more than a process of technological exteriorization at work: it discloses an interior excess as the locus for a dynamic evacuating and inscriptive movement enabling the co-emergence of both humanity and technology. Beings are not only inserted into matrices: matrices are inscribed in beings.