The central question of Michel Foucault’s work The Order of Things (Les mots et les choses. Une archéologie des sciences humaines, 1966) is whether we can consider the world that we perceive as the expression of the essence of things or rather as the result of actions of particular mechanisms that order our perceptions. Foucault abstracts these levels of perception, calling them epistemes. An episteme is a historically changing structure which defines conditions of possibility of beliefs, theories, or sciences in each given historical period. It is a structure of thought expressing the mode of thinking that is proper to a given historical epoch.

Related to the concept of episteme as an organizing principle is the idea of resemblance. Says Foucault,

Up to the end of the sixteenth century, resemblance played a constructive role in the knowledge of Western culture. It was resemblance that largely guided exegesis and the interpretation of texts; it was resemblance that organized the play of symbols, made possible knowledge of things visible and invisible, and controlled the art of representing them.

According to Foucault, there are four types of resemblances, or similitudes, in the world.

The first type of resemblance is convenientia:

A resemblance connected with space in the form of a graduated scale of proximity. It is of the same order as conjunction and adjustment. This is why it pertains less to the things themselves than to the world in which they exist. The world is simply the universal "convenience" of things.

The second type is aemulatio. Through emulation, everything in the world interacts with itself.

The human face, from afar, emulates the sky, and just as man’s intellect is an imperfect reflection of God’s wisdom, so his two eyes, with their limited brightness, are a reflection of the vast illumination spread across the sky by sun and moon . . . The relation of emulation enables things to imitate one another from one end of the universe to the other without connection or proximity.

The third is analogy. This type of similitude is comprised of both convenientia and aemulatio.

In this analogy, convenientia and aemulatio are superimposed. Like the latter, it makes possible the marvellous confrontation of resemblances across space; but it also speaks, like the former, of adjacencies, of bonds and joints.

The fourth is sympathy. This type of similitude draws various things existing in the world to each other. Sympathy is balanced with antipathy. The latter preserves things in isolation from each other and prevents them from identifying with one another. Owing to antipathy, everything in the world remains what it is.

Foucault explores a change in man's form of thinking, or in the so-called epistemological field. An episteme is a level of knowledge proper to a historical period. Knowledge, according to Foucault, is a system of ordering things and of their correlation with words. He singles out three epistemes.

The first is the episteme of the Renaissance (sixteenth century). Words and things are identical. Language is not an independent sign system but rather is dispersed among natural things. Words and things constitute аn integrated text.

The second is that of the Classical age (seventeenth through eighteenth centuries). Words and things interact through the medium of thought. Language becomes an autonomous sign system, which leads to “mathematicization” of knowledge and the birth of such self-contained disciplines as general grammar and natural history.

The third is the modern episteme (since the end of the eighteenth century). Words and things are mediated through language. Language becomes an object of study. It turns into a system of formal elements and displays...