The French philosopher René Descartes declared that all animals except humans are “machines,” very complicated automata, responding to external stimuli in a mechanical way. Humans alone, because they possess immaterial souls, are conscious and endowed with free will and therefore capable of being virtuous or sinful. However, Julien Offroy de La Mettrie denied the existence of any sharp distinction between humans and other animals. In his Histoire naturelle de l’âme (1745; Treatise on the Soul, 1996), he raised a strong objection to Descartes’s calling brutes “machines,” thereby denying that they think and feel. However, three years later, he changed his terminology (not his doctrine) and argued that animals are machines that feel, and so are humans. “The human body is a machine which winds its own springs; . . . the soul is but a principle of motion or a material and sensible part of the brain.” Man a Machine, the better-known translation of L’Homme-machine, is somewhat of a misnomer. Although in English it seems strange to call a man or a dog a machine, it makes sense to speak of them as mechanisms, which is what La Mettrie meant. Hence his philosophy is often referred to as mechanism.

Man a Machine is for the most part a treatise on physiological psychology, containing also certain ethical and antitheological reflections. It is in the form of an oration, without subdivisions. La Mettrie begins with a defense of experience and observation as foundations of knowledge even about the soul, as against the claims put forward for revelation as a source superior to reason. “If there is a God, He is the Author of nature as well as of revelation; . . . if there is a revelation, it can not contradict nature.” Although nature certainly stands in need of interpretation, so does the Bible. Concerning the soul, the requisite “experience and observation . . . are to be found throughout the records of the physicians who were philosophers, and not in the works of the philosophers who were not physicians. . . . Only the physicians have a right to speak on this subject.” According to La Mettrie, theologians have the least right.

What experience and observation show the philosopher-physician about the soul is that its character is patently dependent on bodily conditions. When the body is diseased, so is the soul. A genius may be reduced to idiocy by a fever; it sometimes happens, conversely, that “the convalescence of an idiot produces a wise man.” Extreme bodily fatigue produces a sleep amounting to the temporary extinction of the soul. The effects of opium, wine, and coffee are cited. In addition, diet influences character. The English are savage because they eat their meat red and bloody. In La Mettrie’s opinion, the English diet accounts for their vices of “pride, hatred, scorn of other nations, indocility and other sentiments which degrade the character”—but education can counteract this. Extreme hunger and prolonged sexual abstinence can produce raving maniacs. When the body degenerates in old age, so does the soul. Female delicacy and male vigor correspond to the different bodily constitutions of the sexes. When one looks through a gallery of portraits, one “can always distinguish the man of talent from the man of genius, and often even an honest man from a scoundrel.” Differences in national character correspond to differences in climate. In sum, “the diverse states of the soul are always correlative with those of the body.”

Comparative mammalian anatomy, especially brain anatomy, bears out and explains this conclusion. Humans are the most intelligent of animals because they have the largest and most convoluted brains. The descending order of intelligence—monkey, beaver, elephant, dog, fox, cat—is also the descending order of brain size and complexity. La Mettrie makes three generalizations about animals: “1st, that the fiercer animals are, the less brain they have; 2nd, that this organ seems to increase in size in proportion to the gentleness of the animal; 3rd, that . . . the more one gains in intelligence the more one loses in instinct.” However, among humans, brain defects are not always gross: “A mere nothing, a tiny fibre, something that could never be found by the most delicate anatomy, would have made of Erasmus and Fontenelle two idiots.”

The higher animals can do surprising things when properly trained. It would be interesting to attempt to teach an ape to speak by application of the methods used so brilliantly with deaf-mutes. If one chose a fairly young ape, “one with the most intelligent face, and the one which, in a thousand little ways, best lived up to its look of intelligence,” the experiment might well succeed. Then the ape “would no longer be a wild man, nor a defective man, but he would be a perfect man, a little gentleman, with as much matter or muscle as we have, for thinking and profiting by his education.” For humans are not distinguished qualitatively from the other animals except in possessing language. Language itself is not an inherent possession of the human species as such, but must have been invented by certain geniuses who taught it to the others.

Knowledge consists of the comparison of the sensory ideas (images) produced in the brain, and this comparison can hardly proceed without language, a system of symbols for classifying. This comparison La Mettrie calls “imagination,” and he asserts: “All the faculties of the soul can be reduced to pure imagination. . . . Thus, judgment, reason, and memory are not absolute parts of the soul, but merely modifications of [the] medullary screen upon which images of the objects painted in the eye are projected as by a magic lantern.” Hence, all talent and genius is fundamentally the same thing: lively imagination. “Man’s preeminent advantage is his organism. . . . An exaggerated modesty (a rare fault, to be sure) is a kind of ingratitude towards nature.” There is nothing wrong with taking pride not only in skill, learning, and virtue, but even in mind, beauty, wealth, nobility; these, “although the children of chance, all have their own value.”

It is wrong to try to distinguish humans from other animals by the former’s alleged exclusive acquaintance with natural (moral) law. Natural law is “a feeling that teaches us what we should not do, because we would not wish it to be done to us.” It manifests itself to me when, for example, I feel remorse after bad conduct; my belief that you have a similar experience can only be based on my inferences from your behavior. However, we see the same signs in animals, such as the “crouching and downcast air” of a dog that has offended its master. La Mettrie cites the story of Androcles and the lion to prove that animals feel gratitude. If, however, it is maintained that despite appearances, animals do not really have any awareness of natural law, then it follows that people do not either, for “man is not moulded from a costlier clay; nature has used but one dough, and has merely varied the leaven.” However, in fact, remorse and gratitude are universal, even among the most hardened criminals. These persons commit their atrocities from morbid impulses, and they are punished adequately by their consciences. It would be better to hand them over to doctors than to burn them or bury them alive, as is the custom.

It is clear that for these reasons, virtue is its own reward, and that “Nature has created us all solely to be happy—yes, all of us from the crawling worm to the eagle lost in the clouds.” La Mettrie developed the ethical implications of this doctrine in his Discours sur le bonheur: Ou, L’Anti-Sénèque (1747; Anti-Seneca: Or, The Sovereign Good, 1996).

La Mettrie next turns his attention to religion. It is highly probable, he says, that a supreme being exists; but this is “a theoretic truth with very little practical value.” It does not follow that a highest being ought to be worshiped just because it exists; nor does religion (as everyone knows) ensure morality, any more than atheism excludes it.

The “zealous writers” who pile up evidences of design in nature to prove the existence of an intelligent Creator are misguided. “Either the mere structure of a finger, of an ear, of an eye, a single observation of Malpighi proves all, . . . or all the other evidences prove nothing.” For even if it is admitted that these facts rule out the possibility of a merely “chance” universe, the existence of a supreme being is not thereby proved, “since there may be some other thing which is neither chance nor God—I mean, nature.” All people know is that there is an infinite variety of ingenious mechanisms in nature; people know nothing of their ultimate causes; in this situation, recourse to God is a mere disguise of ignorance. “The weight of the universe therefore far from crushing a real atheist does not even shake him.”

At this point, La Mettrie writes (more astonishingly than convincingly): “Such is the pro and the contra, and the summary of those fine arguments that will eternally divide the philosophers. I do not take either side.” A friend of his, however, “an abominable man,” maintained to him that “the universe will never be happy, unless it is atheistic.” The extirpation of religion would put an end to religious wars. “Nature, infected with a sacred poison, would regain its rights and its purity. Deaf to all other voices, tranquil mortals would follow only the spontaneous dictates of their own being, the only commands which can never be despised with impunity and which alone can lead us to happiness through the pleasant paths of virtue.”

Returning to the subject of the soul, La Mettrie next shows that it is not necessary to postulate the soul as a principle or cause of motion of the body because muscular fiber is inherently motile. He offers ten observations and experiments in proof. One is that portions dissected from polyps regenerate into whole polyps; the other nine are concerned either with spontaneous motions of parts of organisms severed from bodies or with the motion of parts of the body after death. “The soul is therefore but an empty word, of which no one has any idea, and which an enlightened man should use only to signify the part in us that thinks.”

La Mettrie describes at considerable length the physiology of reflex and involuntary movements to illustrate the mechanical nature of the body. He cites, among other things, the phenomenon of erection. The bodily effects of emotional states show, moreover, that there is no sharp division between what is under control of the will and what is not. Though La Mettrie does not deny (or even discuss) the “freedom of the will,” he remarks that the will “cannot act save by permission of the bodily conditions.” However, having shown (to his satisfaction) that the body is self-moved and that consciousness is a property of its organized matter, not an independent substance, La Mettrie confesses that he can go no further in explanation. “The nature (origin) of motion is as unknown to us as that of matter.”

La Mettrie praises Descartes for having proved that animals are machines. His insistence on the distinctness of mental from material substance was, according to La Mettrie, a ruse to throw the theologians off the scent; for the analogy of humans with animals is so striking that it could only be overlooked by “animals and machines which, though upright, go on all fours.” There is no contradiction in the notion of a thinking, feeling, moral animal-machine. “Thought is so little incompatible with organized matter, that it seems to be one of its properties on a par with electricity, the faculty of motion, impenetrability, extension, etc.” Only pride and prejudice lead people to resist these conclusions. However, “matter contains nothing base, except to the vulgar eyes which do not recognize her in most splendid works.” La Mettrie again states that immortality is not impossible. To suppose it out of the question would be to reason like caterpillars who can have no conception of their coming metamorphosis. People should admit that they are invincibly ignorant in this domain and will remain so.

La Mettrie concludes by picturing the wisdom, justice, tranquillity, reverence (for nature), gratitude, affection, tenderness, kindliness, pity, and forgiveness—in a word, the happiness—of the materialist.Convinced, in spite of the protests of his vanity, that he is but a machine or an animal, the materialist will not maltreat his kind, for he will know too well the nature of those actions, whose humanity is always in proportion to the degree of the analogy proved between human beings and animals; and following the natural law given to all animals, he will not wish to do to others what he would not wish them to do to him. Let us then conclude boldly that man is a machine, and that in the whole universe there is but a single substance differently modified. . . . Such is my system, or rather the truth, unless I am much deceived. It is short and simple. Dispute it now who will.

More than two centuries after La Mettrie, one is likely to smile wryly at the pretty picture of grateful lions regulating their conduct by the Golden Rule, and at the conviction that once religion is gone, all will be well—as if religion were something imposed on people from outside, contrary to “the spontaneous dictates of their own being.” However, these amiable eighteenth century ideas should not deceive anyone into supposing that La Mettrie was a naïve thinker. He deserves the credit (or blame) for many insights usually attributed to such philosophers as Jean-Jacques Rousseau, Étienne Bonnot de Condillac, Claude-Adrien Helvétius, and Paul-Henri-Dietrich d’Holbach. His brief remarks on the relation of evidence to conclusion in the design argument for the existence of God penetrate to the essential logical point in a manner not inferior to the more celebrated critiques of David Hume and Immanuel Kant.

Materialists, at least since Lucretius, argued that no chasm separates humans from the rest of nature and that the soul must be material, or a property of matter, because mental states obviously vary with the condition of the body. La Mettrie did not have a new argument; he only added to the old one such evidence as was available to him from recent investigations of brain anatomy. However, just as La Mettrie questioned the relevance of piling up evidence for design in nature for the purpose of proving a Great Designer, so here also one can ask whether more and more detail about the dependence of soul on body strengthens the conclusion that soul is body. Except for theological considerations (which La Mettrie justifiably ignored), philosophers who were well aware of all the facts concerning the effects of bodily constitution on the soul still upheld the separateness of body and soul for three reasons: First, one’s thoughts and feelings, of which one is directly aware, are obviously neither bodies nor properties of bodies; it makes no sense to raise questions about them as to where they are or how big they are. Second, one knows that the self has an identity that no material thing, or property, could have, for anything that occupies space can be divided, whereas one has no notion of what it would be like to split oneself into two selves. Third, matter is essentially inert; if it moves, there must be a cause of its motion; and one’s experience reveals that volition, which is mental, is capable of moving the body.

La Mettrie paid attention to the third objection; he gave good reasons for denying the inertness of matter, especially organic matter. An answer to the second is implicit in his writing: The unity of the self is only a unity of functions, and when the organism is malformed (as in congenital idiocy or deafness), the corresponding functions (“faculties”) are absent.

There is discernible in La Mettrie the bare beginning of a materialist reply to the first objection. He says that “judgment, reason, and memory are not absolute parts of the soul, but merely modifications of [the] medullary screen upon which images of the objects painted in the eye are projected as by a magic lantern.” That is, when an image is formed on the retina, it is transmitted by the optic nerve to the visual cortex, and the visual sensation is the resulting “modification” or “brain-event.” This doctrine requires considerable argumentation and explication before it becomes plausible, and La Mettrie provides none. However, he has at any rate progressed beyond the view of Descartes, according to whom the immaterial soul somehow “inspects” (directly, infallibly, and unintelligibly) the “medullary screen.” In any case, it would be unreasonable to complain of La Mettrie that he did not, once and for all, explain how one is to conceive the identity of thought and brain process—that is, solve the mind-body problem, still one of the most vexing questions on the philosophical agenda.

La Mettrie was the first, the most consistent, and the most extreme of the eighteenth century French materialists. He was a thinker of great originality who insisted on expressing his thoughts in print, well knowing that to do so would expose him to the rage of fanatical obscurantists. In fact, he was forced to flee from France to Holland, thence to Prussia, where Frederick II granted him asylum. In Potsdam, he resumed the practice of medicine. He enjoyed but two years of security and prosperity; he died at the age of forty-one. (The pious claimed that epicurean gluttony was the cause of death.) Frederick himself composed his eulogy, saying of him:La Mettrie was born with a fund of natural and inexhaustible gaiety; he had a quick mind, and such a fertile imagination that it made flowers grow in the field of medicine. Nature had made him an orator and a philosopher; but a yet more precious gift which he received from her was a pure soul and an obliging heart. All those who are not imposed upon by the pious insults of the theologians mourn in La Mettrie a good man and a wise physician.