Jeffrey Bell, in another one of his superb readings of Spinoza (or, more precisely, perhaps, of Deleuze’s Spinoza), discusses “Eternity and Duration”, by which he also means the difference between the virtual/problematic (which he associates with Spinoza’s substance) and the actual/determinate (which he associates with Spinoza’s modes). Bell says that, in Spinoza,
the human Mind that is eternal is not the determinate, identifiable mind, but rather the immanent condition for the possibility of such a determinate identification; it is, in short, the infinite power of self-ordering becoming (the ‘infinite enjoyment of existing’) that allows for the possibility of determinate, singular bodies, and for the determinate singular minds that are the ideas of these bodies.
This means — to give a crude reduction of Bell’s argument — that Spinoza’s mind/substance/God is equivalent to Deleuze’s virtual; it is an immanent potentiality. Any actual mind/body is a particular finite determination or actualization of that potentiality (a “solution” to that problematic). There is a continual movement from the problematic — “what can a body do?” — to particular actualizations, or to “modifications and affections of determinate bodies and minds,” that in effect instantiate or realize this problematic. And conversely, there is a counter-movement from the actual back to the virtual, due to the fact that “our determinate bodies and minds require the problematic as the ‘infinite enjoyment of existing’.” The ethical movement in Spinoza, and implicitly in Deleuze as well, is this countervailing movement “from the actual and determinate, from what this body is actually doing or has done, to the problematic and the virtual, the body as an eternity that is not to be confused with the determinate and which is indeed subject to many variations and which we can never fully possess.” This is how we attain Spinoza’s third kind of knowledge, or more generally the freedom that is the subject of Book 5 of The Ethics. Bell’s reformulation clarifies for me both how this works in Spinoza (against the initial impression that Book 5 is merely a retreat to conventional morality after the bold metaphysics and psychology of Books 1-4), and how central this all is to Deleuze’s own vision of the virtual, and indeed of liberation.
But I want to add an important point to this, by adding Whitehead to the discussion. For Whitehead never offers us such a movement back to the virtual as we find in Spinoza and in Deleuze. Indeed, Whitehead specifically declares himself to be inverting Spinoza in this crucial regard. In Whitehead’s own philosophy, “Spinoza’s ‘modes’ now become the sheer actualities; so that, though analysis of them increases our understanding, it does not lead us to the discovery of any higher grade of reality… In such monistic schemes [as Spinoza’s], the ultimate is illegitimately allowed a final, ’eminent’ reality, beyond that ascribed to any of its accidents” (PR 7). In Whitehead’s resolutely pluralistic ontology, on the other hand, there are only modes or affections, the actual occasions. There is no substance, nothing behind the modes or affections, for them to be modes or affections of. This is because of Whitehead’s effort to get us away from “subject-predicate forms of thought.”
Nearly all the Spinozists and Deleuzians I know would reject Whitehead’s account as a misreading of Spinoza, a claim that Spinozian substance, or God (Deus sive Natura) is somehow transcendent, when in fact it is entirely immanent. (Bell promises to explain in a subsequent post how Spinoza’s third kind of knowledge, or his ascent from the actual back to the virtual, can “be understood in a way that doesn’t reintroduce transcendence”). However, I want to suggest that Whitehead is right. Even if it escapes transcendence, Spinozian substance is still a subject for all the predicates, a monism behind the pluralism. Whitehead, by his own admission, offers a philosophy that “is closely allied to Spinoza’s scheme of thought.” But if Whitehead does not quite set Spinoza on his feet (as Marx claimed to set Hegel on his feet, and as Deleuze claimed that Nietzsche had set Kant on his feet), he does unhinge Spinoza (in the way that, according again to Deleuze, Kant unhinges the classical notion of time, or casts it, in Shakespearean parlance, out of joint). He does this by dethroning substance, or — to put the matter back into Bell’s formulations with which I started this posting — by in a certain sense deprivileging the virtual, or at least rejecting the ethical priority of the virtual in Spinoza (and in Deleuze as well).
One can see this most clearly, I believe, by contrasting Whitehead’s God with Spinoza’s God. Whitehead secularizes God (PR 207) more radically and extensively than Spinoza does; Whitehead’s God, like Spinoza’s — and also like Deleuze/Guattari’s “body without organs,” as I argued in my book — is indeed associated with the virtual rather than the actual; but for this reason, God in Whitehead is curiously marginalized (as Substance in Spinoza is not). God operates for Whitehead as a sort of repository of the virtual, in that he envisages all “eternal objects” or potentialities indiscriminately (this is the “primordial” nature of God). God also functions as a sort of Bergsonian memory, in which all the past is preserved (this is the “consequent” nature of God). But by decentering God, and by splitting him up in this manner, Whitehead disallows anything like a return (a re-ascent?) back to the virtual from the actual. In this way, Spinoza’s third kind of knowledge is for Whitehead a kind of idealist illusion that needs to be rejected: the point being that it is still idealist, even if it is entirely immanent and doesn’t imply any recourse to transcendence. (A similar criticism is implied of Bergson, or at least of that side of Bergson that Deleuze also draws upon in his account of returning from the actual to the virtual. The primordial nature of God is Whitehead’s revision of Spinoza, and the consequent nature of God is Whitehead’s revision of Bergson; in both cases, Whitehead brings us further than Deleuze ever dares to).
If we speak of the virtual, instead of God, then the point is that Whitehead’s often-rejected (even by his admirers) theory of potentialities as “eternal objects” should be seen as a secularization of theories of the virtual such as we find in Deleuze (with its roots in both Spinoza and Bergson). To put the matter very quickly (there is a more extended discussion in my book; but doubtless this is also something that I will need to work out more fully  andcarefully): Every actual entity constitutes itself by a decision that accepts certain eternal objects, while rejecting others. The eternal objects that “ingress” into any actual entity are something like its predicates or qualities; except that no entity can be defined as just the sum of its predicates or qualities, because it is not just a collocation of characteristics (which would be to return to “subject-predicate forms of thought”). Rather, no list of an actual entity’s qualities can give us the entity, because such a list excludes a crucial dimension: the entity as process, or the way in which it selects, and then organizes or “harmonizes”, those qualities. This added dimension is a process or an action, rather than anything substantial (this is where I diverge somewhat from Graham Harman’s admirable notion of “allure,” as the dimension of an object that is withdrawn from, and in excess of, all its qualities).
For Whitehead, therefore, in consonance with Deleuze and Spinoza, something like the virtual or the potential needs to be determined or actualized. This actualization is the process of an actual entity (or, as Whitehead also calls it, an actual occasion) terminating in something absolutely determinate. But there is no movement back from the determinate to the virtual. Rather, once something is determinate, it perishes; and what has perished subsists as a “datum” for new determinations, which themselves, in taking up the data that precede them, must once again actualize potentiality.. and so on, ad infinitum. The movement from the virtual (potentiality, eternal objects) to the actual is involved with and necessary to, but it is also somewhat lateral or oblique to, the most crucial movement in Whitehead’s cosmology, which goes from perished entities (“data”) to new entities, which perish in their own term and thus provide data to new entities, etc.
In this way, I think, Whitehead avoids the Deleuzian suggestion (which one also finds in Bergson, and — in Bell’s reading — already in Spinoza, and currently in the wonderful neo-Schellingism of Iain Hamilton Grant) that the actual must always (with this “must” being something of an ethical imperative) return to the flux of virtuality whence it came. In this way, Whitehead is in accordance with Graham Harman (who rejects the association of Whitehead with Deleuze and Bergson precisely on these grounds). But, to the extent that Whitehead does nonetheless retain the importance of the virtual, he also stands apart from Harman’s actualism. My biggest objection to Harman has long been that he doesn’t give a sufficiently satisfying account of the genesis and perishing of objects, precisely because he rejects the very notion of the virtual, seeing it as something that “undermines” the existence of objects. Whitehead to my mind splits the difference between Deleuze and Harman, in a way that is preferable to either. (Note: I cannot end this discussion without an apology to Levi Bryant, who offers a version of “object-oriented ontology” that includes the virtual. I think that Whitehead represents a preferable alternative to Bryant’s position as well, in the sense that he obviates the need to see objects as somehow being “withdrawn.” But I do not have the space or the energy to pursue this argument here).