The vast terrain that is philosophy is populated by conceptual personae, Deleuze asserts. A conceptual persona is a partially or entirely fictional character that enters a philosophical text to proclaim certain ideas. As Nietzsche taught, ideas can not be thought by everyone in an equal manner, nor can all ideas emerge from everyone. Certain people are needed to think certain ideas. Weak ideas can only emerge from weak dispositions of character, and strong ideas can only emerge from the strong. And thus, philosophers call all sorts of characters on to the stage of their text to make possible the emergence of ideas. There are Plato's Ideas, which can only arise from the questioning gadfly that is Socrates, there is Descartes' Cogito that can only arise from the Idiot Eudoxus, and the message of Zarathustra is only possible by grace of the dialectical clash of characters that is Also sprach Zarathustra. Character determines concept, and conceptual personae are philosophy's way of injecting this truth of life into the activity of philosophy.
Concepts have a certain coherence among themselves, but this coherence is granted by the problems and questions to which concepts respond. The concept of the Cogito only gains sense because of the problem of certainty, expressed by the questions of the Meditations. And the concept of the Idea only gains sense because of the problem of being, expressed by the questioning of Socrates. To understand the concept, we have to understand the problem from which it arises. And problems do not only arise from other concepts, but from life, from a concrete life. Socrates' life in the Athenian demos gives rise to the question "who is the true statesman?", and Socrates' life among sophists gives rise to the question "who is the true philosopher?" Conceptual personae bridge this gap between the problems of concrete life, and the concepts of philosophy. Conceptual personae manifest a life, traversed by problems, from which certain questions arise. Questions to which concepts respond.
Is this why Nietzsche was so excited about Dostoevsky? This greatest psychologist drove to its climax the idea that lives are needed for ideas. In Dostoevsky, ideas are never proclaimed as ideas, but as expressions of the different lives that his texts lay out. The nihilist, the ridiculous man, the gambler, the underground man, each has his own ideas. Ideas which are only comprehensible from the intricacies of their lives. Idea and life are interwoven in such a manner that speaking of any distinction between the two is void of meaning. The thoughts of the underground are expressions of the situation of the underground. Perceived from the question of the genesis of ideas, every situation is in essence problematic. That is, a situation constitutes a problem from which ideas arise. Ideas which respond to the question that arises from the confrontation with a certain situation. There are ideas that can only arise from a Gulag, and there are ideas that can only arise from the experience of madness.
Problems lay down the rules by which concepts must abide, they lay down the playing field. Like the Kantian Ideas determine the edges of the island of Reason, beyond which concepts lose sense, because they no longer correspond to problems. But in reality, unlike in Kant, problems are not universal Ideas, but concrete problematic situations that perplex a thinker to such a degree that he feels the need to respond to the situation with an idea. When the conceptual activity of philosophy remains solely on the level of concepts and their possibility, and doesn't entertain the question of the emergence of concepts out of life, the living problem cannot be grasped. Yet philosophy, because of the specificity of its activity, is necessarily stuck on the plane of concepts and their purely conceptual coherence. The philosophical text has no room for the intricate genesis of an idea out of life, a room which is accessible for a writer like Dostoevsky. But the philosopher does have conceptual personae, they bridge the gap between idea and life, and are able to show the problematic situations from which concepts arise.
These conceptual personae make sense of the problem or position from which a concept can emerge. They elucidate the ways of living and thinking that make possible different concepts. What way of thinking makes possible the enunciation of the Cogito? This is the life of Eudoxus, which expresses a way of thinking that characterizes itself as an Idiot, an idiot in the face of scholasticism and its conceptual inventory, unaware of the traditions, this character is able to think for and from himself and himself only. Just like the writer of fiction, the philosopher needs characters.
But why? Is the philosopher, by himself, not enough for ideas to emerge? Why does he need the intermediary of a conceptual persona, that acts between himself and the concept? The answer might be that he does not need the character, or rather, that he himself can function as the persona. But this is not always the case. Some concepts can only be expressed through highly dramatized positions, persona that act as highly condensed expressions of specific problems. Like Descartes' Eudoxus. It is highly unrealistic to pretend to entirely think for oneself, and thus one needs a persona. So too Berkeley's Euphranor, it is highly unlikely to find a person that balances the virtues of physicality and thought to a perfect degree, and thus one needs the persona of Euphranor; the farmer who expresses immaterialism. Conceptual personae act as ideals. Ideals, not necessarily as morally good things to strive for, but simply as idealized summits of activity. End-goals, of whatever way one may choose to live or think. Socrates, like the commentaries of late antiquity show like no other, as the ideal of philosophy as problematization. Eudoxus as the ideal of free thought, and Dostoevsky's underground man as the ideal of violence in thought. Characters explain concepts, and concepts express characters.