The Foucauldian genealogy is an unmasking of power for the use of those who suffer it. It is also directed against those who would seize power in their name. As Francois Ewald pints out, there are three, not two, parties to every power struggle: not only those who exercise power and those who would exercise it in their place, but also those on whom it is exercised. Because one speaks against power, one does not necessarily speak with those who suffer it. Hence Foucault's concerted attack on all forms of interpretation and representation: on the use made of the linguistics of Saussure and Jakobson, on psychoanalysis, on Marxism. For the interpreter, things are never what they seem. People never say what they mean or mean what they say; they never know what they want or what they are doing.
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But Foucault shows that truth does not exist outside power, still less in opposition to it. Each society has its own regime of truth: the types of discourse accepted as true, the mechanisms that make it possible to distinguish between truth and error. ... The will to the power of truth is a pitiless tyrant; it requires a singular and total devotion. It is a service that has tempted the European mind since Plato. Nietzsche gave the first signs of its possible end; he also provided a way out, which is called genealogy. Genealogy was a 'grey' activity, but it was also a gay science, a science of the hypothetical. That gaiety, that love of hypothesis, pervades all Foucault's work. He is the reverse of a guru, a teacher, a subject who is supposed to know, though he would, in all modesty, be flattered if, without excessive seriousness, he were compared to a Zen master, who knows nothing. For him uncertainlty causes no anguish:...
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All of the above is a quote from Alan Sheridan's Foucault The Will To Truth (p. 219-224)
For a genealogy of the movie Eclipse see here.