The idea of sense is pretty important for all of Foucault’s work, but it first emerges as a notion which historically transforms psychology. In the psychology of the 1800s, you see an approach to pathology which uses largely the same models as physiological pathology. Phenomena (symptoms), including behaviours, are bundled together statistically based on the phenomena with which they’re usually observed, that type of thing, to get an idea of the disease. Later, especially with Freud and Husserl (although it was only possible because of Janet), the idea of sense is introduced. In this context, sense is best explained by looking at Freud. In Freud, we see that the symptom (say, hallucination) is a repetition of a defence mechanism established during childhood in response to anxiety (an ambivalence of feelings, contradictory desires). When such anxiety occurs again in adulthood, the defence mechanism is reiterated (for developmental reasons this is the first time it can be called a defence mechanism, but whatever). The symptom is not simply an occurrence, it is a behaviour that is undertaken with a goal, namely, to avoid anxiety. Such a goal is, of course, unconscious. The crucial point is that the symptom ceases to be a simple phenomenon, it gains something like a direction, an aiming-at. In Freud this isn’t fully developed, but that’s not important for our purposes, what’s important is that separating the behaviour from its sense is no longer theoretically viable. Over time, this has spread into all sorts of different fields: economics, for example, no longer deals simply with transactions, but also with the sense those transactions have.
At this point, it’s worth noting that when I say “sense” I’m translating the French word “sens”, which has an important ambiguity. On the one hand, “sens” can be a word for “direction”. On the other hand, “sens” can be translated as “meaning” (as in, “this word means…”). The English “sense” has something of this ambiguity, but not to the same degree. In the context we’re talking about, it’s both, the point is that the two cannot be distinguished. The fact that it comes from psychology is important because it deals not only with the outcome towards which a behaviour tends, but also with what it means to the person doing it. In Freud this is the importance of symbolism; a behaviour initially used to escape the anxiety of the threat of castration can be a defence mechanism for adult anxiety only when the source of the adult anxiety is read, symbolically, as a form of castration. To take an example from phenomenological psychology, in Binswanger the fact that a patient reports feeling like they are from a different planet makes sense only when coupled with the fact that their construction of the world is in some way pathological, they literally are part of a different world (I could go into the difference between Freudian and post-Freudian symbolism, but that would be a tangent). The two translations of “sens”, “meaning” and “direction”, are inseparable.
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The reason this matters for Foucault’s work in general is that he never gives up on this notion. Obviously he stops using it as an explanation of psychopathological phenomena (mostly), but sense retains a crucial position in his work. In the archaeological works, a statement (énoncé) has sense, in that the meaning of the statement is inseparable from how it alters the discursive formation within which it takes its place. In the works on power, every act is shaped by power relations, an act gets its sense from its relation to the ensemble of power relations, but it also produces a change in this ensemble (this is how resistance is possible; on the other hand, if an act reaffirms power relations, it still changes them simply by virtue of the fact that it is a repetition, so see Deleuze). In the ethical works, an act has sense by changing the norms according to which the individual’s behaviour is conducted. Sense is always essential, because it is what allows a single, individual phenomenon to have a relationship with an apparatus without either side of this relationship determining the essence of the other, and without the relationship being dialectical.