“Replacing essences with social conventions or subjective beliefs is a relatively safe move, but putting in their  place a new set of objective entities involves philosophical speculation. What guides this speculation? One way of looking at this question is to see Deleuze as engaged in a constructive project guided by certain prescriptive constraints, that is, constraints which tell him not what to do but what to avoid doing. One such constraint is, of course, to avoid the trap of essentialism […]” (pp. 29-30)

“Meeting this constraint requires rejecting much of what modal logic has to say about possibility and necessity. The reason is that the postulation of possible worlds existing alongside the actual world, as Quine and other critics have often remarked, almost always implies commitment to one or another form of essentialism. And, it should be emphasized, this criticism applies not only to modal philosophers but also to those physicists who seriously believe in the existence of alternate parallel universes.

When thinking about these parallel universes, both philosophers and physicists assume the existence of fully formed individuals populating the different possible worlds. This immediately raises a number of questions: Can the same individual exist, slightly altered, in other worlds? Can he or she maintain this identity across many worlds, after several slight alterations have accumulated? Could we identify him or her after all these changes? It is here that essences, either general or particular, are introduced to define the identity of these individuals and to guarantee its preservation across worlds. […]

The alternative offered by Deleuze is to avoid taking as given fully formed individuals, or what amounts to the same thing, to always account for the genesis of individuals via a specific individuation process, such as the development process which turns an embryo into an organism. This emphasis on the objective production of the spatio-temporal structure and boundaries of individuals stands in stark contrast with the complete lack of process mediating between the possible and the real in orthodox modal thinking. […] As Deleuze writes:

What difference can there be between the existent and the non-existent if the non-existent is already possible, already included in the concept and having all the characteristics that the concept confers upon it as a possibility? … The possible and the virtual are … distinguished by the fact that one refers to the form of identity in the concept, whereas the other designates a pure multiplicity … which radically excludes the identical as a prior condition […] Actualization breaks with resemblance as a process no less than it does with identity as a principle. In this sense, actualization or differenciation* is always a genuine creation […]

Besides the avoidance of essentialist thinking, Deleuze’s speculation is guided by the closely related constraint of avoiding typological thinking, that style of thought in which individuation is achieved through the creation of classifications and of formal criteria for membership in those classifications. Although some classifications are essentialist, that is, use transcendental essences as the criterion for membership in a class, this is not always the case. For example, unlike Platonic essences which are transcendent entities, Aristotle’s ‘natural states’, those states towards which an individual tends, and which would be achieved if there were no interfering forces, are not transcendent but immanent to those individuals. But while Aristotelian philosophy is indeed non-essentialist it is still completely typological, that is, concerned with defining the criteria which group individuals into species, and species into genera. […]

[Biological taxonomies common to 17th & 18th century Europe] were supposed to reconstruct a natural order which was fixed and continuous, regardless of the fact that historical accidents may have broken that continuity. In other words, given the fixity of the biological types, time itself did not play a constructive role in the generation of types, as it would later on in Darwin’s theory of the evolution of species.

Deleuze takes the four elements which inform these classificatory practices, resemblance, identity, analogy and opposition (or contradiction) as the four categories to be avoided in thinking about the virtual. Deleuze, of course, would not deny that there are objects in the world which resemble one another, or that there are entities which manage to maintain their identity through time. It is just that resemblances and identities must be treated as mere results of deeper physical processes, and not as fundamental categories on which to base an ontology. Similarly, Deleuze would not deny the validity of making judgments of analogy or of establishing relations of opposition, but he demands that we give an account of that which allows making such judgments or establishing those relations. And this account is not to be a story about us, about categories inherent in our minds or conventions inherent in our societies, but a story about the world, that is, about the objective individuation processes which yield analogous groupings and opposed properties.

[…] In addition to showing, case by case, how similarity and identity are contingent on the details of an individuation process, the rejection of static categories and essences must be extended to all natural kinds, not just biological ones.” (pp. 39-43)

[*Deleuze distinguishes b/w differenTiation (“the progressive unfolding of a multiplicity through broken symmetries”) & differenCiation (“the progressive specification of the continuous space formed by multiplicities as it gives rise to our world of discontinuous spatial structures”), p.23.]