Deleuze used the term virtual to refer to an aspect of reality that is ideal, but nonetheless real. An example of this is the meaning, or sense, of a proposition that is not a material aspect of that proposition but is nonetheless an attribute of that proposition. Both Henri Bergson, who strongly influenced Deleuze, and Deleuze himself build their conception of the virtual in reference to a quotation in which writer Marcel Proust defines a virtuality, memory as "real but not actual, ideal but not abstract". A dictionary definition written by Charles Sanders Peirce, referencing the philosophy of Duns Scotus, supports this understanding of the virtual as something that is "as if" it were real, and the everyday use of the term to indicate what is "virtually" so, but not so in fact.
Deleuze's concept
Deleuze's concept of the virtual has two aspects: first, the virtual is a kind of surface effect produced by actual causal interactions at the material level. When one uses a computer, the monitor displays an image that depends on physical interactions happening at the level of hardware. The window is nowhere in actuality, but is nonetheless real and can be interacted with. This example actually leads to the second aspect of the virtual that Deleuze insists upon: its generative nature. This virtual is a kind of potentiality that becomes fulfilled in the actual. It is still not material, but it is real. Deleuze argues that Henri Bergson developed "the notion of the virtual to its highest degree" and that he based his entire philosophy on it. In Bergsonism, Deleuze writes that "virtual" is not opposed to "real" but opposed to "actual", whereas "real" is opposed to "possible". This definition, which is almost indistinguishable from potential, originates in medieval Scholastics and the Medieval Latin word virtualis. Deleuze identifies the virtual, considered as a continuous multiplicity, with Bergson's "duration": "it is the virtual insofar as it is actualized, in the course of being actualized, it is inseparable from the movement of its actualization."
Other concepts
Another core meaning has been elicited by Denis Berthier, in his 2004 book "", based on uses in science, technology, and etymology. At the same ontological level as "the possible" abstractions, representations, or imagined "fictions", the actually-real "material", or the actually-possible "probable", the "virtual" is "ideal-real". It is what is not real, but displays the full qualities of the real—in a plainly actual —way. The prototypical case is a reflection in a mirror: it is already there, whether or not one can see it; it is not waiting for any kind of actualization. This definition allows one to understand that real effects may be issued from a virtual object, so that our perception of it and our whole relation to it, are fully real, even if it is not. This explains how virtual reality can be used to cure phobias. Brian Massumi shows the political implications of this. However, note that the writers above all use terms such as "possible", "potential" and "real" in different ways and relate the virtual to these other terms differently. Deleuze regards the opposite of the virtual as the actual. Rob Shields argues that the opposite of the virtual is the material for there are other actualities such as a probability. The virtual is far more than a technical or communications term. Martin Luther argued in his writing The Sacrament of the Body and Blood of Christ—Against the Fanatics with other Protestants, most notably Zwingli, over the virtualism of the Christian Eucharist, in alignment with Catholic tradition, that the Eucharist was actually and not virtually the body and blood of Christ. According to Massumi in "Parables for the Virtual", the virtual is something "inaccessible to the senses" and can be felt in its effects. His definition goes on to explain virtuality through the use of a topological figure, in which stills of all of the steps in its transformation superposed would create a virtual image. Its virtuality lies in its inability to be seen or properly diagramed, yet can be figured in the imagination.
