Gilles Deleuze, answered from the texts and cited to the page.
The question of virtual and actual is not, first of all, a question about representation — and that displacement matters. Let me say why. The virtual is not opposed to the real; it is the real that is opposed to the possible.1 This is the confusion that ruins most thinking on the subject: people assume the virtual is somehow less than actual, a kind of shadow or sketch awaiting completion.
The possible operates exactly this way — it is constructed retrospectively, in the image of the real it supposedly precedes, so that realization adds nothing to it except bare existence.2 The possible is the concept of the thing plus the predicate "exists"; the virtual is something altogether different in kind. The virtual belongs to the Idea and does not resemble the actual.3
Actualization is always creative with respect to what it actualizes — it proceeds not by resemblance and limitation but by divergence and differentiation.4 When certain biologists speak of organic potentiality being actualized by simple limitation of a global capacity, they fall precisely into this confusion: they treat the virtual as a possible, something realized by exclusion rather than created by positive differentiation.5
To actualize the virtual, one must create lines of actualization that did not pre-exist the process — difference is primary, not resemblance.6 Now, there are two movements to distinguish. In the first, the actual is surrounded by increasingly extensive and diverse virtualities — a perception evokes memories, a particle creates ephemera, and the virtual circles widen.7
In the second, the circles contract: the virtual draws so close to the actual that they become less and less distinct, until you reach an inner circuit linking only the actual object and its virtual image.8 Memory, as Bergson shows, is not an actual image formed after perception but a virtual image coexisting with the actual perception — its double, simultaneous, contemporary with the object rather than subsequent to it.9
At this inner circuit, something further happens: the exchange between virtual and actual becomes perpetual, and what results is not actualization but crystallization.10 The crystal is precisely this — a regime in which one can no longer assign "actual" and "virtual" to distinct objects, because the two have become indiscernible.11 The virtual image never stops becoming actual; the actual character is no more than a virtuality.
This connects to the most fundamental split in time itself. The actual is defined by the passing of the present — a variable given, measured in continuous, supposedly mono-directional time.12 The virtual, by contrast, operates at two scales simultaneously: it is ephemeral, occurring in a space of time smaller than the smallest continuous movement in a single direction, and yet it is also the longest time, preserving the past in all directions.13
The passing of the present and the preservation of the past are the two jets of time — and actual and virtual are their respective images. During actualization, these remain distinguishable though with unassignable limits; during crystallization, they exchange roles to the point of becoming indiscernible.14 So to your question about representation: the virtual cannot be represented, because representation works through resemblance and identity — the very categories the virtual undoes.
The Idea is an image without resemblance.15 What the virtual calls for is not representation but expression — the creation of lines of differentiation adequate to its own divergent movement.
The virtual not opposed to the real; it is the real that is opposed to the possible. Virtual sa full reality.Desert Islands and Other Texts 1953-1974, p. 100
the possible because of this defect that the possible looks suspiciously retrospective or retroaspected of being constructed after the fact, in resemblance to the real pposed to precede.Desert Islands and Other Texts 1953-1974, p. 100
the virtual belongs to the Idea and does not resemble the actual, no more than the actual resembles the virtual.Desert Islands and Other Texts 1953-1974, p. 100
Differentiation or actualization is always creative with respect to what it actualizes, whereas realization is always reproductive or limiting.Desert Islands and Other Texts 1953-1974, p. 100
When certain biologists invoke a notion of organic virtuality or potentiality and nonetheless maintain that this potentiality is actualized by simple limitation of its global capacity, they clearly fall into a confusion of the virtual and the possible.Bergsonism, pp. 98–99
the characteristic of virtuality is to exist in such a way that it is actualized by being differentiated and is forced to differentiate itself, to create its lines of differentiation in order to be actualized.Bergsonism, pp. 98–99
we have considered those cases in which the actual is surrounded by increasingly extensive, remote and diverse virtualities: a particle creates ephemera, a perception evokes memories.Dialogues II Revised Edition, pp. 177–178
the inverse movement also occurs: in which, as the circles contract, the virtual draws closer to the actual, both become less and less distinct.Dialogues II Revised Edition, pp. 177–178
memory is not an actual image which forms after the object has been perceived, but a virtual image coexisting with the actual perception of the object. Memory is a virtual image contemporary with the actual object, its double, its 'mirror image'.Dialogues II Revised Edition, pp. 177–178
This is no longer a singularization, but an individuation as process, the actual and its virtual: no longer an actualization but a crystallization.Dialogues II Revised Edition, pp. 177–178
It is not so much that one cannot assign the terms 'actual' and 'virtual' to distinct objects, but rather that the two are indistinguishable.Dialogues II Revised Edition, pp. 178–179
The present is a variable given measured in continuous time, a supposedly mono-directional movement, in which the present passes up until the exhaustion of that time. The actual is defined by this passing of the present.Dialogues II Revised Edition, pp. 178–179
The period of time which is smaller than the smallest period of continuous time imaginable in one direction is also the longest time, longer than the longest unit of continuous time imaginable in all directions.Dialogues II Revised Edition, pp. 178–179
The two aspects of time, the actual image of the present which passes and the virtual image of the past which is preserved, are distinguishable during actualization although they have unassignable limits, but exchange during crystallization to the extent that they become indiscernible.Dialogues II Revised Edition, pp. 178–179
The Idea is an image without resemblance; the virtual actualizes itself not through resemblance, but through divergence and differentiation.Desert Islands and Other Texts 1953-1974, p. 100