Of all the couples in the history of modern thought (Freud and Lacan, Marx and Lenin…), Kant and Sade is perhaps the most problematic: the statement "Kant is Sade" is the "infinite judgement" of modern ethics, positing the sign of equation between the two radical opposites, i.e. asserting that the sublime disinterested ethical attitude is somehow identical to, or overlaps with, the unrestrained indulgence in pleasurable violence. A lot-everything, perhaps-is at stake here: is there a line from Kantian formalist ethics to the cold-blooded Auschwitz killing machine? Are concentration camps and killing and genocides as a neutral business the inherent outcome of the enlightened insistence on the autonomy of Reason? Is there at least a legitimate lineage from Sade to Fascist torturing, as is implied by Pasolini's film version of Saló, which transposes it into the dark days of Mussolini's Saló republic? Lacan developed this link first in his Seminar on [[Books/Jacques_Lacan/The_Seminar_Of_Jacques_Lacan_The_Ethics_Of_Psychoanalysis|"The Ethics of Psychoanalysis"] (1958-59),  and then in the Écrits "[[Books/Jacques_Lacan/Kant_With_Sade/ Kant with Sade]" of 1963. 
For Lacan, Sade consequently deployed the inherent potential of the Kantian philosophical revolution, in the precise sense that he honestly externalized the Voice of Conscience. The first association here is, of course: what's all the fuss about? Today, in our postidealist Freudian era, doesn't everybody know what the point of the "with" is the truth of Kant's ethical rigorism is the sadism of the Law, i.e. the Kantian Law is a superego agency that sadistically enjoys the subject's deadlock, his inability to meet its inexorable demands, like the proverbial teacher who tortures pupils with impossible tasks and secretly savors their failings?
Lacan's point, however, is the exact opposite of this first association: it is not Kant who was a closet sadist, it is Sade who is a closet Kantian. That is to say, what one should bear in mind is that the focus of Lacan is always Kant, not Sade: what he is interested in are the ultimate consequences and disavowed premises of the Kantian ethical revolution. In other words, Lacan does not try to make the usual "reductionist" point that every ethical act, as pure and disinterested as it may appear, is always grounded in some "pathological" motivation (the agent's own long-term interest, the admiration of his peers, up to the "negative" satisfaction provided by the suffering and extortion often demanded by ethical acts); the focus of Lacan's interest rather resides in the paradoxical reversal by means of which desire itself (i.e. acting upon one's desire, not compromising it) can no longer be grounded in any "pathological" interests or motivations and thus meets the criteria of the Kantian ethical act, so that "following one's desire" overlaps with "doing one's duty." Suffice it to recall Kant's own famous example from his Critique of Practical Reason:
Suppose that someone says his lust is irresistible when the desired object and opportunity are present. Ask him whether he would not control his passions if, in front of the house where he has this opportunity, a gallows were erected on which he would be hanged immediately after gratifying his lust. We do not have to guess very long what his answer may be."3 Lacan's counterargument here is: what if we encounter a subject (as we do regularly in psychoanalysis), who can only fully enjoy a night of passion if some form of "gallows" is threatening him, i.e. if, by doing it, he is violating some prohibition? 
There was an Italian film from the 60's, Casanova 70, starring Virna Lisi and Marcello Mastroianni that hinged on this very point: the hero can only retain his sexual potency if doing "it" involves some kind of danger. At the film's end, when he is on the verge of marrying his beloved, he wants at least to violate the prohibition of premarital sex by sleeping with her the night before the wedding-however, his bride unknowingly spoils even this minimal pleasure by arranging with the priest for special permission for the two of them to sleep together the night before, so that the act is deprived of its transgressive sting. What can he do now? In the last shot of the film, we see him crawling on the narrow porch on the outside of the high-rise building, giving himself the difficult task of entering the girl's bedroom in the most dangerous way, in a desperate attempt to link sexual gratification to mortal danger… So, Lacan's point is that if gratifying sexual passion involves the suspension of even the most elementary "egotistic" interests, if this gratification is clearly located "beyond the pleasure principle," then, in spite of all appearances to the contrary, we are dealing with an ethical act, then his "passion" is stricto sensu ethical… 
Lacan's further point is that this covert Sadean dimension of an "ethical (sexual) passion" is not read into Kant by our eccentric interpretation, but is inherent to the Kantian theoretical edifice.  If we put aside the body of "circumstantial evidence" for it (isn't Kant's infamous definition of marriage-"the contract between two adults of the opposite sex about the mutual use of each other's sexual organs"-thoroughly Sadean, since it reduces the Other, the subject's sexual partner, to a partial object, to his/her bodily organ which provides pleasure, ignoring him/her as the Whole of a human Person?), the crucial clue that allows us to discern the contours of "Sade in Kant" is the way Kant conceptualizes the relationship between sentiments (feelings) and the moral Law.
Although Kant insists on the absolute gap between pathological sentiments and the pure form of moral Law, there is one a priori sentiment that the subject necessarily experiences when confronted with the injunction of the moral Law, the pain of humiliation (because of man's hurt pride, due to the "radical Evil" of human nature); for Lacan, this Kantian privileging of pain as the only a priori sentiment is strictly correlative to Sade's notion of pain (torturing and humiliating the other, being tortured and humiliated by him) as the privileged way of access to sexual jouissance (Sade's argument, of course, is that pain is to be given priority over pleasure on account of its greater longevity-pleasures are passing, while pain can last almost indefinitely). This link can be further substantiated by what Lacan calls the Sadean fundamental fantasy: the fantasy of another, ethereal body of the victim, which can be tortured indefinitely and nonetheless magically retains its beauty (see the standard Sadean figure of a young girl sustaining endless humiliations and mutilations from her deprived torturer and somehow mysteriously surviving it all intact, in the same way Tom and Jerry and other cartoon heroes survive all their ridiculous ordeals intact).
Doesn't this fantasy provide the libidinal foundation of the Kantian postulate of the immortality of the soul endlessly striving to achieve ethical perfection, i.e., is not the phantasmic "truth" of the immortality of the soul its exact opposite, the immortality of the body, its ability to sustain endless pain and humiliation?
Judith Butler pointed out that the Foucaultian "body" as the site of resistance is none other than the Freudian "psyche": paradoxically, "body" is Foucault's name for the psychic apparatus insofar as it resists the soul's domination. That is to say, when, in his well-known definition of the soul as the "prison of the body," Foucault turns around the standard Platonic-Christian definition of the body as the "prison of the soul," what he calls "body" is not simply the biological body, but is effectively already caught into some kind of pre-subjective psychic apparatus.  Consequently, don't we encounter in Kant a secret homologous inversion, only in the opposite direction, of the relationship between body and soul: what Kant calls "immortality of the soul" is effectively the immortality of the other, ethereal, "undead" body?
It's via this central role of pain in the subject's ethical experience that Lacan introduces the difference between the "subject of the enunciation" (the subject who utters a statement) and the "subject of the enunciated (statement)" (the symbolic identity the subject assumes within and via his statement): Kant does not address the question of who is the "subject of the enunciation" of the moral Law, the agent enunciating the unconditional ethical injunction-from within his horizon, this question itself is meaningless, since the moral Law is an impersonal command "coming from nowhere," i.e. it is ultimately self-posited, autonomously assumed by the subject himself). Via the reference to Sade, Lacan reads absence in Kant as an act of rendering invisible, of "repressing," the moral Law's enunciator, and it is Sade who renders it visible in the figure of the "sadist" executioner-torturer-this executioner is the enunciator of the moral Law, the agent who finds pleasure in our (the moral subject's) pain and humiliation.
A counterargument offers itself here with apparent self-evidence: isn't all this utter nonsense, since, in Sade, the element that occupies the place of the unconditional injunction, the maxim the subject has to follow categorically, is no longer the Kantian universal ethical command Do your duty! but its most radical opposite, the injunction to follow to their utmost limit the thoroughly pathological, contingent caprices that bring you pleasure, ruthlessly reducing all your fellow humans to the instruments of your pleasure? However, it is crucial to perceive the solidarity between this feature and the emergence of the figure of the "sadist" torturer-executioner as the effective "subject of the enunciation" of the universal ethical statement-command. The Sadean move from Kantian Respect-to-Blasphemy, i.e. from respecting the Other (fellow being), his freedom and autonomy, and always treating him also as an end-in-itself, to reducing all Others precisely to mere dispensable instruments to be ruthlessly exploited, is strictly correlative to the fact that the "subject of the enunciation" of the Moral Injunction, invisible in Kant, assumes the concrete features of the Sadean executioner.
What Sade accomplishes is thus a very precise operation of breaking up the link between two elements which, in Kant's eyes, are synonymous and overlapping:  the assertion of an unconditional ethical injunction; the moral universality of this injunction. Sade keeps the structure of an unconditional injunction, positing as its content the utmost pathological singularity.
And, again, the crucial point is that this breaking up is not Sade's eccentricity-it lays dormant as a possibility in the very fundamental tension constitutive of the Cartesian subjectivity. Hegel was already aware of this reversal of the Kantian universal into the utmost idiosyncratic contingency: isn't the main point of his critique of the Kantian ethical imperative that, since the imperative is empty, Kant has to fill it with some empirical content, thus conferring on contingent particular content the form of universal necessity?
The exemplary case of the "pathological," contingent element elevated to the status of an unconditional demand is, of course, an artist absolutely identified with his artistic mission, pursuing it freely without any guilt, as an inner constraint, unable to survive without it. The sad fate of Jacqueline du Pré confronts us with the feminine version of the split between the unconditional injunction and its obverse, the serial universality of indifferent empirical objects that must be sacrificed in the pursuit of one's Mission.  (It is extremely interesting and productive to read du Pré's life story not as a "real story," but as a mythical narrative: what is so surprising about it is how closely it follows the preordained contours of a family myth, the same as with the story of Kaspar Hauser, in which individual accidents uncannily reproduce familiar features from ancient myths.) Du Pré's unconditional injunction, her drive, her absolute passion was her art (when she was 4 years old, upon seeing someone playing a cello, she already immediately claimed that this is what she wanted to be…). This elevation of her art to the unconditional relegated her love life to a series of encounters with men who were ultimately all substitutable, one as good as the other-she was reported to be a serial "man eater." She thus occupied the place usually reserved for the MALE artist-no wonder her long tragic illness (multiple sclerosis, from which she was painfully dying from 1973 to 1987) was perceived by her mother as an "answer of the real," as divine punishment not only for her promiscuous sexual life, but also for her "excessive" commitment to her art…
This, however, is not the whole story. The decisive question is: is the Kantian moral Law translatable into the Freudian notion of superego or not? If the answer is yes, then "Kant with Sade" effectively means that Sade is the truth of the Kantian ethics. If, however, the Kantian moral Law cannot be identified with superego (since, as Lacan himself puts it in the last pages of [[Books/Jacques_Lacan/The_Seminar_Of_Jacques_Lacan_The_Four_Fundamental_Concepts_Of_Psychoanalysis|"The Four Fundamental Concepts of Psycho-Analysis"], moral Law is equivalent to desire itself, while superego precisely feeds on the subject's compromising his/her desire, i.e. the guilt sustained by the superego bears witness to the fact that the subject has somewhere betrayed or compromised his/her desire),  then Sade is not the entire truth of Kantian ethics, but a form of its perverted realization. In short, far from being "more radical than Kant," Sade articulates what happens when the subject betrays the true stringency of the Kantian ethics.
This difference is crucial in its political consequences: insofar as the libidinal structure of "totalitarian" regimes is perverse (the totalitarian subject assumes the position of the object-instrument of the Other's jouissance), "Sade as the truth of Kant" would mean that Kantian ethics effectively harbors totalitarian potentials; however, insofar as we conceive of Kantian ethics as precisely prohibiting the subject to assume the position of the object-instrument of Other's jouissance, i.e. to calling on him to assume full responsibility for what he proclaims his Duty, then Kant is the antitotalitarian par excellence…
The dream about Irma's injection that Freud used as the exemplary case to illustrate his procedure of analyzing dreams is a dream about responsibility-(Freud's own responsibility for the failure of his treatment of Irma)-this fact alone indicates that responsibility is a crucial Freudian notion.
But how are we to conceive it? How are we to avoid the usual trap of the mauvaise foi of the Sartrean subject responsible for his existential project, i.e. of the existentialist motif of ontological guilt that pertains to the finite human existence as such, as well as the opposite trap of "putting the blame on the Other" ("since the Unconscious is the discourse of the Other, I am not responsible for its formations, it is the big Other who speaks through me, I am merely its instrument…")?
Lacan himself pointed the way out of this deadlock by referring to Kant's philosophy as the crucial antecedent of the psychoanalytic ethics of the duty "beyond the Good". According to the standard pseudo-Hegelian critique, the Kantian universalist ethic of the categorical imperative fails to take into account the concrete historical situation in which the subject is embedded, and which provides the determinate content of the Good: what eludes Kantian formalism is the historically specified particular Substance of ethical life. However, this reproach can be countered by claiming that the unique strength of Kant's ethics resides in this very formal indeterminacy: moral Law does not tell me what my duty is, it merely tells me that I should accomplish my duty, i.e. it is not possible to derive the concrete norms I have to follow in my specific situation from the moral Law itself-which means that the subject himself has to assume the responsibility of "translating" the abstract injunction of the moral Law into a series of concrete obligations.
In this precise sense, one is tempted to risk a parallel with Kant's Critique of Judgement: the concrete formulation of a determinate ethical obligation has the structure of aesthetic judgement, i.e. of a judgement by which, instead of simply applying a universal category to a particular object or of subsuming this object under an already given universal determination, I as it were invent its universal-necessary-obligatory dimension and thereby elevate this particular-contingent object (act) to the dignity of the ethical Thing.
So there is always something sublime about pronouncing a judgement that defines our duty: in it, I "elevate an object to the dignity of the Thing" (Lacan's definition of sublimation). The full acceptance of this paradox also compels us to reject any reference to "duty" as an excuse: "I know this is heavy and can be painful, but what can I do, this is my duty…" The standard motto of ethical rigor is "There is no excuse for not accomplishing one's duty!"; although Kant's Du kannst, denn du sollst! ("You can, because you must!") seems to offer a new version of this motto, he implicitly complements it with its much more uncanny inversion: "There is no excuse for accomplishing one's duty!"  The reference to duty as the excuse to do our duty should be rejected as hypocritical; suffice it to recall the proverbial example of a severe sadistic teacher who subjects his pupils to merciless discipline and torture. Of course, his excuse to himself (and to others) is: "I myself find it hard to exert such pressure on the poor kids, but what can I do-it's my duty!" The more pertinent example is that of a Stalinist politician who loves mankind, but nonetheless performs horrible purges and executions; his heart is breaking while he is doing it, but he cannot help it, it's his Duty towards the Progress of Humanity…
What we encounter here is the properly perverse attitude of adopting the position of the pure instrument of the big Other's Will: it's not my responsibility, it's not me who is effectively doing it, I am merely an instrument of the higher Historical Necessity… The obscene jouissance of this situation is generated by the fact that I conceive of myself as exculpated for what I am doing: isn't it nice to be able to inflict pain on others with the full awareness that I'm not responsible for it, that I merely fulfill the Other's Will…this is what Kantian ethics prohibits. This position of the sadist pervert provides the answer to the question: How can the subject be guilty when he merely realizes an "objective", externally imposed necessity? By subjectively assuming this "objective necessity," i.e. by finding enjoyment in what is imposed on him. So, at its most radical, Kantian ethics is NOT "sadist," but precisely what prohibits assuming the position of a Sadean executioner.
In a final twist, Lacan thus nonetheless undermines the thesis of "Sade as the truth of Kant." It is no accident that the same seminar in which Lacan first deployed the inherent link between Kant and Sade also contains the detailed reading of Antigone in which Lacan delineates the contours of an ethical act that DOES successfully avoid the trap of the Sadean perversion as its hidden truth-in insisting on her unconditional demand for her brother's proper burial, Antigone does NOT obey a command that humiliates her, a command effectively uttered by a sadistic executioner… So the main effort of Lacan's seminar on the Ethics of Psychoanalysis is precisely to break up the vicious cycle of Kant avec Sade. How is this possible? Only if-in contrast with Kant-one asserts that the faculty of desiring is not in itself "pathological." In short, Lacan asserts the necessity of a "critique of pure desire": in contrast to Kant, for whom our capacity to desire is thoroughly "pathological" (since, as he repeatedly stresses, there is no a priori link between an empirical object and the pleasure this object generates in the subject), Lacan claims that there is a "pure faculty of desire," since desire does have a non-pathological, a priori object-cause-this object, of course, is what Lacan calls objet petit a.
 Lacan, Jacques, Le séminaire, Livre VII: L'éthique de la psychanalyse, Paris: Seuil, 1986, chap. VI.
 Lacan, J., "Kant avec Sade," in Écrits, Paris: Seuil, 1966, p. 765-790.
 Kant, Immanuel, Critique of Practical Reason, New York: Macmillan, 1993, p. 30.
 /…/ if, as Kant claims, no other thing but the moral law can induce us to put aside all our pathological interests and accept our death, then the case of someone who spends a night with a lady even though he knows that he will pay for it with his life, is the case of the moral law." Alenka Zupančič, "The Subject of the Law," in Cogito and the Unconscious, ed. by Slavoj Žižek, Durham: Duke UP 1998, p. 89.
 he most obvious proof of the inherent character of this link of Kant with Sade, of course, is the (disavowed) Kantian notion of "diabolical Evil," i.e. of Evil accomplished for no "pathological" reasons, but out of principle, just for the sake of it." Kant evokes this notion of Evil elevated into a universal maxim (and thus turned into an ethical principle) only in order to disclaim it immediately, claiming that human beings are incapable of such utter corruption; however, shouldn't we counter this Kantian disclaimer by pointing out that de Sade's entire edifice relies precisely on such an elevation of Evil into an unconditional ("categorical") imperative? For a closer elaboration of this point, see Chapter Chapter II of Slavoj Žižek, "The Indivisible Remainder", London: Verso 1996.
 Butler, Judith, The Psychic Life of Power, Stanford: Stanford University Press 1997, p. 28-29.
 David-Menard, Monique, Les constructions de l'universel, Paris: PUF 1997.
 Du Pré, Hilary and Piers, A Genius in the Family. An Intimate Memoir of Jacqueline du Pré, London: Chatto and Windus 1997.
 Alenka Zupančič, op.cit., as well as Bernard Baas, Le désir pur, Louvain: Peeters 1992.
 For a more detailed account of this key feature of Kant's ethics, see Chapter II of Slavoj Žižek, "The Indivisible Remainder", London: Verso 1996.