From a review of Sartre’s War Diaries:
Sartre is not observing this fellow human being, but adapting him, making of him an instant philosophical villain.
A very common motif in writing about philosophers is a certain image of the relation between ‘ideas’ and ‘life’. The usual story is that the philosopher will use people and situations in order to ‘illustrate’ ideas, i.e., they, philosopher, do not see people in their immediacy but see through them to abstract intellectual preoccupations. This common line of thought posits as it own presupposition a curious notion of what ‘ideas’ are. (And what 'life' is). Ideas are already assumed to be as abstract, removed from life and impractical as the philosophers who think them.
This is a lazy and impoverished notion.
What Deleuze says he encounters in Michel Foucault is not, directly, a person, but a ‘set of chiselled sounds’, ‘abrupt conclusions’, various speeds and movements, an austere and controlled violence, impersonal particles grouped together under a proper name. Foucault’s gestures and movements constellate a new concept for Deleuze – that of ‘pre-personal singularities’. In other words, the encounter with Foucault is at the same time the encounter with certain ideas in their immediate form.
Foucault is not someone Deleuze overwrites with ideas – that it, someone who Deleuze colonises and colours-in with his own ideas, erasing the specifics of Foucault himself. Foucault himself is the colonisation and disturbance of Deleuze. A disturbance of Deleuze’s own ideas, a disturbance which then leaves an open space, on open space in which thought might breath.
Ideas are not something fabulated in the laboratory of the head and then projected onto a rather mundane, otherwise neutral 'life'; rather are ideas immediately present in the encounter with people and objects. They are immanent in the world and ambush us from the outside.