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.

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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...