Michel Foucault's book The Order of Things constitutes

an inquiry whose aim is to rediscover on what basis knowledge and theory became possible.

Toward that end, Foucault traces the evolution, in tandem, of "knowledge and theory" through the ancient Greek "classical" moment, through the Renaissance, into modernity and, finally, contemporaneity. His argument is that the history of episteme (conceived of as a temporally specific constellation of material and intellectual preconditions allowing for the pursuit and acceptance of some—but not all—kinds of knowledge) is not a history of "growing perfections" but rather of "that of its conditions of possibility."

In other words, the history of a given mode of knowledge (with attendant conceptions of theory) is not a history of expansion and enrichment, but rather of the material circumstances that made such a mode propitious and attractive at a given time. For example, engineering science as a mode of knowledge is only possible once society has been industrialized. What we can know depends upon the age in which we live, as well as such factors as our geographical location and socioeconomic status (and resultant access to education). Historical, material conditions are not merely "influential"; rather, they are constitutive. Thus, Foucault's work is strenuously historicist.

The results of his historical determinism are readily apparent in remarks such as

Marxism exists in nineteenth-century thought like a fish in water: that is, it is unable to breathe anywhere else.

The clarity and forcefulness of this claim are heightened by Foucault's use of the "fish in water" metaphor. Yet upon closer examination, Marxism is arguably more like a rug woven out of the threads of Hegelianism, Locke's labor theory of value, Christianity, and nineteenth-century social science, none of which was empirically grounded. Thus, the rhetorical impact of the claim about Marxism as a breathing organism does not hold up well upon historical examination of the philosophical sources of Marxism.

So certain is Foucault of his claims that he tends to make not only sweeping claims but also to offer rhetorical consolation to those readers whom he imagines will be discomfited by his claims:

It is comforting, however, and a source of profound relief to think that man is only a recent invention, a figure not yet two centuries old, a new wrinkle in our knowledge, and that he will disappear again as soon as that knowledge has discovered a new form.

This is true enough, in terms of the particular Pico della Mirandola–derived conception of "man" that Foucault has in mind, but as far as some anthropomorphically specific conception of homo sapiens goes, it is much less clear that we can forecast its disappearance. Foucault depends upon an almost scientific notion of historicism: everything disappears on schedule as conditions change. But his own notion of history as a disappearance-machine is of dubious historicity; for example, why should Renaissance notions of "man" and, by extension, "human dignity," still be so influential today? And yet, if we refer to developments only in twenty-first-century international law, we see a massive growth in the influence of notions of "human dignity" that are traceable to the the Renaissance and even late antiquity.

Foucault's historicism does not prevent him from making psychological statements that are apparently meant to apply to all persons of a certain type over time:

From the point of view of wealth, there is no difference between need, comfort and pleasure.

How can we say that this is true of the "point of view of wealth" in all times and places? Should we not differentiate between wealth in one place at one time and in another?

Foucault's combination of rhetorical certitude and intellectual ambivalence make for stimulating reading; the internal contradictions of his work are consistent with his roots in Nietzschean and Baconian philosophy.