In a particularly difficult passage (pages 58 ff) in Order of Things, Foucault argues that Descartes engages in a critique of resemblance (in his Regulae). Instead of the orthodox argument, in which “sixteenth century thought becom[es] troubled as it contemplates itself”, Descartes excludes resemblance as a fundamental experience and primary form of knowledge. Descartes accomplishes this by universalizing the act of comparison in rational thought, thereby giving it its purest form. The challenge is, however, that according to Descartes true knowledge can only arise from intuition (as an act of “pure and attentive intelligence”) linked through deduction, but this excludes comparison almost by definition. Comparison exists in only two forms, but must be reconfigured: the comparison of measurement and of order. Measurement analyzes the world into units that establish relations of equality and inequality. Order analyses elements, the simplest possible that can be found, and arranges differences according to the smallest possible degrees. With Descartes, classical resemblance ceased to be the fundamental category of knowledge, and instead became an analysis of identity and difference.
The “progress” of the method is such that measurement is reduced to serial arrangement (as an act of order), which shows up in differences of degrees of complexity. This analysis progresses from the unit and relations of equality and inequality to an analysis of identity and differences (“differences that can be thought in the order of inferences”). This analysis of identity and difference no longer fulfilled its role in revealing how the world is ordered, since it now progresses according to the “order laid down by thought”. In the 16th century, kinships, resemblances and affinities, which are interwoven with thought, take on a new configuration, which can be summed up as “rationalism” (“if one’s mind is filled with ready-made concepts”). In the classical episteme knowledge was never complete and always open to fresh possibilities, based on similitude. The new system of comparison permits a “complete enumeration” with certain knowledge of identity and differences, as each point can be necessarily connected to the next. In this new system the action of the mind will no longer draw things together (establishing kinship, affinity, etc.), instead it will act by discriminating (establishing identities and making successive series of connections). Finally, history and science will become separated, because there is no common unit of measurement. Language, thus, is no longer one of the “figurations of the world”, and while it can translate truth if it can, it can no longer be considered the “mark of it”.