The concept of the simulacrum gained currency in critical writing about Postmodernism. For Frederic Jameson, for example, it was part of Postmodernism’s new “depthlessness”, (see surface / depth) and its weakening of historicity into a world of images — of copies with no originals. The simulacrum “simulates reference.” Jameson’s well-known example was Photorealism, which at first looked like a return to representation and figuration…until it became clear that its objects were not to be found in the “real world”, but were themselves photographs, making the “realism” of the photorealist painting into a simulacrum. (Postmodernism, p.30) It is this “derealization” of the real word which Jameson identifies with the simulacrum and its role in Late Capitalism — in which “the past itself has disappeared,” and the image has become “the final form of commodity reification” (Debord).

Jean Baudrillard would celebrate postmodern reality as hyperreality: a condition in which the real and the imaginary are confused. According to Baudrillard, when it comes to postmodern simulation and simulacra, “It is no longer a question of imitation, nor duplication, nor even parody. It is a question of substituting the signs of the real for the real.” Thus, the "The territory no longer precedes the map, nor does it survive it. It is the map that precedes the territory— precession of simulacra, in his terms —that engenders the territory" (see Christian Hubert, “The Ruins of Representation”. For Baudrillard, postmodern reality is hyperreality: a condition in which the real and the imaginary are confused. 

But the history of the simulacrum extends back into the Platonic discussions of representation. For Plato and Aristotle, simulation is directly linked to the real. For Plato, the “transcendent Idea” provides a method for hierarchical ranking by degrees of resemblance to a pure, perfect, and transcendent original. Plato’s motive for the theory of Ideas is distinguish between essence and appearance, to select, to sort out, to distinguish between the “thing” itself, and its images, the original and the copy, the model and the simulacrum. (Deleuze) It is a matter of choosing between claimants, of distinguishing the good from the false copies. Iconic copies are authorized by resemblance, but phantasmatic simulacra are like false claimants, implying a perversion. They are corrupted by dissemblance. The simulacrum is at the bottom rung of this hierarchy, in which any hint of resemblance to the idea disappears. (Ryan Johnson)

But for Deleuze, the simulacrum still produces an effect of resemblance and can contribute to the “overthrow of Platonism” through it phantasmic power, its repressed power. “To overthrow Platonism means: to raise up simulacra, to assert their rights over icons or copies” (p.52) It is a Dionysiac machine, with the power of affirming chaos.

Lucretius’ accounts differ fundamentally from Plato’s. He describes a physical process that can and often does mislead the mind into false beliefs, but Lucretius does not situate this characteristic within a discussion of truth or representation. What Deleuze calls Lucretius’ “naturalism” speaks only of nature, rather than mythic gods or transcendent forms. It actively eliminates from philosophy any hint of transcendence, myth, or superstition that would render nature less real, secondary, or negative. Lucretian atomism affirms the full immanence and productive power from within. Even the philosophical position of naturalism is understood as a natural product.

In Lucretius’ atomistic account of the simulacrum, the continual vibration of the component atoms of objects causes atoms on the surface to be thrown off constantly at high speed, as extraordinarily fine films shaped like the objects from which they emanate. When these emanations and emissions, these filmy images (most often called eidola in Greek and simulacra in Latin) impinge upon the eyes, they produce sight; and when they enter the mind, they cause thought — if the body is awake — or dreams if it is asleep. (Martin Ferguson Smith, pp xxvii-xxviii) Like the clinamen, simulacra occur in a time more fleeting than the minimum of perceived, continuous time — “as quick as thought” (Epicurus). They are not real objects, although they have a reality. They are empty husks, rigid envelopes that only retain a form. Their emission is a symptom of the body’s continual reproduction of itself qua representation. (Holmes) “We are immersed in simulacra; it is through them that we perceive, that we dream, that we desire, and that we act.” (Deleuze)

The work of both Epicurean physics and Epicurean ethics is the struggle to eliminate false beliefs, to distinguish what is really infinite in nature from false infinity. The simulacra introduce error, not in themselves, but in the reactions we have to them — in our reactions that attribute properties to the absolute sensible object that belong to the simulacra instead. It is through this false infinity that we introduce the image of infinity into our desires and our images of fear and punishment into infinity itself. As Deleuze puts it “False infinity is the principle of the soul’s disturbance”.

For the Epicureans, the main examples of the soul’s disturbance are found in loving desire — the belief in the possibility of infinite pleasure — and in the belief in the immortal soul, with its capacity for infinite pain. Erotic desire is incapable of possessing its real object. It can only enjoy simulacra and knows bitterness and torment in its frustrated desire to be infinite. These illusions capture us through our own fears and desires: the more we are governed by the hope of infinite pleasure and the fear of eternal punishment, the more we focus on those phantasms that confirm and strengthen our expectations.

sources/ further reading

Gilles Deleuze, “Lucretius and Naturalism” [1961], translated by Jared C. Bly

Gilles Deleuze, “Lucretius and the Simulacrum” [1969] appendix to the Logic of Sense.

Gilles Deleuze, “Plato and the Simulacrum”, October, volume 27 (Winter 1983) translated by Rosalind Krauss.

Brooke Holmes, “Deleuze, Lucretius, and the Simulacrum of Naturalism”, in: Dynamic Reading: Studies in the Reception of Epicureanism edited by Brooke Holmes, W. H. Shearin.

Ryan J. Johnson, Deleuze-Lucretius Encounter, Edinburgh University Press.n "The Structuralist Activity," of 1960, Roland Barthes defines structure as a simulacrum of the object in which something new occurs: the simulacrum is "intellect added to the object," making something appear which remained invisible, or if one prefers, unintelligible. For Barthes, "Structural man takes the real, decomposes it, then recomposes it." 

Poststructuralist thinkers sever the link between the natural object and its imitation. 
For them, a simulacrum is a copy without an original. 

Thus, the function of the simulacrum is to effect the derealization of the surrounding world of everyday reality. (Jameson) Jameson describes photo-realist art as a simulacrum. It seemed to be about a return of realism after the hegemony of abstraction, but its obects were not "real," but rather images

"Produced by a code or linguistic form that does not really refer but merely simulates reference, the simulacrum is the image or the reproduction that testifies to the short-circuiting of reference in postmodernism." (Simpson, Time, Technology, and the Conversations of Modernity.), p.138) For Simpson, "The simulacrum is the form that value takes in postmodernism. Simpson distinguishes value from meaning. The latter is inseparable from practice (cf. praxis) whereas the former becomes a subjective matter of will.

Is a computer program a simulacrum? 

Is the virtual a state of simulacrum? In Plato, ideas and simulacra exist in some state of virtuality

In her influential " Cyborg Manifesto" Donna Haraway calls microelectronics the technical basis of simulacra. (p. 165)