The philosophical pursuit of metaphysical "order" is highly contested. Typically, Modernists and Theological Philosophers tend to describe the world in terms of its order, and Post-Modernists, Logical Positivists, and New Atheists tend to describe the world purely in terms of accident. Notice that "order" has to do with deep philosophical implications—if it exists, it must be transcendental in nature, since it exists in an infinite, metaphysical way. Therefore, the conversation about "epistemes" must be understood in the context of this ongoing philosophical question (which goes back at least as far as Plato's forms). When put in the proper context, the complicated-sounding ideas become more simple.
This book's core argument seems to be responding to this philosophical question: "If the universe has metaphysical order, then why do cultures vary so wildly from one another? Why shouldn't order dictate what kinds of cultures can exist?" The answer that Foucault offers is basically that metaphysical order is in fact governing cultures, but in ways that circumvent human perception. Foucault points to the "episteme" as a conceptual answer to this question (consider sociology as an example): An "episteme" in sociology would refer to the currency of ideas among all the humans in their respective cultures, especially the ideas they take for granted.
Returning to the beginning of his work with this idea in hand helps to clarify Foucault's original points about science, economics, and biology. In the domain of science, a culture's willingness to use science will depend on their assumptions concerning truth (religious truth, that is). Another culture might find that their economy doesn't work (Foucault would say because their assumptions about economy are not productive).
