Deleuze's main philosophical project in the works he wrote prior to his collaborations with Guattari can be baldly summarized as an inversion of the traditional metaphysical relationship between identity and difference. Traditionally, difference is seen as derivative from identity: e.g., to say that "X is different from Y" assumes some X and Y with at least relatively stable identities (as in Plato's forms). To the contrary, Deleuze claims that all identities are effects of difference. Identities are neither logically nor metaphysically prior to difference, Deleuze argues, "given that there exist differences of nature between things of the same genus." That is, not only are no two things ever the same, the categories we use to identify individuals in the first place derive from differences. Apparent identities such as "X" are composed of endless series of differences, where "X" = "the difference between x and x'", and "x'" = "the difference between...", and so forth. Difference, in other words, goes all the way down. To confront reality honestly, Deleuze argues, we must grasp beings exactly as they are, and concepts of identity (forms, categories, resemblances, unities of apperception, predicates, etc.) fail to attain what he calls "difference in itself." "If philosophy has a positive and direct relation to things, it is only insofar as philosophy claims to grasp the thing itself, according to what it is, in its difference from everything it is not, in other words, in its internal difference."
Like Kant and Bergson, Deleuze considers traditional notions of space and time as unifying forms imposed by the subject. He therefore concludes that pure difference is non-spatio-temporal; it is an idea, what Deleuze calls "the virtual". (The coinage refers to Proust's definition of what is constant in both the past and the present: "real without being actual, ideal without being abstract.") While Deleuze's virtual ideas superficially resemble Plato's forms and Kant's ideas of pure reason, they are not originals or models, nor do they transcend possible experience; instead they are the conditions of actual experience, the internal difference in itself. "The concept they [the conditions] form is identical to its object." A Deleuzean idea or concept of difference is therefore not a wraith-like abstraction of an experienced thing, it is a real system of differential relations that creates actual spaces, times, and sensations.
Thus, Deleuze at times refers to his philosophy as a transcendental empiricism, alluding to Kant and Schelling. In Kant's transcendental idealism, experience only makes sense when organized by forms of sensibility (namely, space and time) and intellectual categories (such as causality). Assuming the content of these forms and categories to be qualities of the world as it exists independently of our perceptual access, according to Kant, spawns seductive but senseless metaphysical beliefs (for example, extending the concept of causality beyond possible experience results in unverifiable speculation about a first cause). Deleuze inverts the Kantian arrangement: experience exceeds our concepts by presenting novelty, and this raw experience of difference actualizes an idea, unfettered by our prior categories, forcing us to invent new ways of thinking (see below, Epistemology).
Simultaneously, Deleuze claims that being is univocal, i.e., that all of its senses are affirmed in one voice. Deleuze borrows the doctrine of ontological univocity from the medieval philosopher John Duns Scotus. In medieval disputes over the nature of God, many eminent theologians and philosophers (such as Thomas Aquinas) held that when one says that "God is good", God's goodness is only analogous to human goodness. Scotus argued to the contrary that when one says that "God is good", the goodness in question is exactly the same sort of goodness that is meant when one says "Jane is good". That is, God only differs from us in degree, and properties such as goodness, power, reason, and so forth are univocally applied, regardless of whether one is talking about God, a person, or a flea.
Deleuze adapts the doctrine of univocity to claim that being is, univocally, difference. "With univocity, however, it is not the differences which are and must be: it is being which is Difference, in the sense that it is said of difference. Moreover, it is not we who are univocal in a Being which is not; it is we and our individuality which remains equivocal in and for a univocal Being." Here Deleuze at once echoes and inverts Spinoza, who maintained that everything that exists is a modification of the one substance, God or Nature. For Deleuze, there is no one substance, only an always-differentiating process, an origami cosmos, always folding, unfolding, refolding. Deleuze summarizes this ontology in the paradoxical formula "pluralism = monism".
Difference and Repetition (1968) is Deleuze's most sustained and systematic attempt to work out the details of such a metaphysics, but his other works develop similar ideas. In Nietzsche and Philosophy (1962), for example, reality is a play of forces; in Anti-Oedipus (1972), a "body without organs"; in What Is Philosophy? (1991), a "plane of immanence" or "chaosmos".