[In the following excerpt, Reed examines the essence and impact of Darwin's contribution to the alternative psychological theory referred to as fluid materialism—a belief that the human mind, and indeed life itself, can be understood within the framework of natural science.]

… Erasmus Darwin's Zoonomia (the first edition appeared in 1792-94, but many subsequent editions were published all over Europe) launched a true alternative psychology, later popularized in his poem The Temple of Nature (1803). In Zoonomia, Darwin defined ideas as the motions of fibers in our organs of sense, and the pattern of these motions. In an addendum to the text he went so far as to say that the theory that ideas are uniquely mental events and not part of everyday nature is nothing but a ghost story. Darwin was a medical doctor by training and a member of the rising bourgeoisie. A kind of no-nonsense attitude characterized his science, even when it was expressed in heroic couplets.

A second key innovation by Darwin was related to his novel definition of ideas. He claimed that the immediate objects of thought are the movements of the relevant neural fibers. The mind does not first make contact with a feeling or sensation and then formulate ideas; rather, the mind simply is the body and all its feelings. If ideas are any and all embodied feelings (not merely cerebral impressions of bodily feelings), then at any one time we may have a large number of different ideas. Erasmus Darwin's concept of the mind, unlike the traditional theory of the soul, allowed that the mind could be multiple, even divided against itself, just as the body can be. Darwin was happy to accept laws of association (among other psychological principles), but he saw associations as determined by the bodily mechanisms, not by the ideas themselves, and certainly not by the transcendent soul. Darwin debunked all such “ghost stories” and set out to show how diseases could be classified according to their disruptive effects on our bodily machinery, including our neural associations. Most diseases were psychosomatic, he thought, and could be cured by diet, exercise, and healthy living—which included adequate access to noncoercive sexual intercourse, especially for women. This was progressive medical advice for his day. It is notable that sin does not enter the picture. Defenders of orthodoxy certainly noted this, and duly attacked Darwin as much for what he did not say as for what he did.

It was largely fear of the implications of Erasmus Darwin's theory that gave rise to the insistence that the causes of our ideas (mental states) could not be known. The young Thomas Brown … lambasted fluid materialism in his popular Observations on the Zoonomia of Erasmus Darwin (1798, p. xx). “The systems of materialism,” Brown wrote, “chiefly owe their rise to the groundless belief, that we are acquainted with the nature of causation.” Whereas the materialists and mentalist opponents can agree that sentience exists, “the mentalist acknowledges, that he is ignorant of the nature of that which causes his ideas; … the materialist, on the contrary, maintains that he is conscious, not merely of ideas, but of the nature of that which causes his ideas.” Again, to use Reid's terms, we sense pricks but perceive thorns—supposedly, we are directly aware of the feeling of being pricked but only infer the existence of thorns. Of course, if we cannot know what causes our ideas, then the story of psychology as a science will be a very short one—yet paradoxically, much of the “new” and ostensibly scientific psychology of the nineteenth century arose in defense of the theory of indirect perception. Reid, who had insisted that we could directly perceive the causes of our ideas, had flatly stated that no scientific analysis of that capacity was possible. The nineteenth-century scientific psychologists insisted that we cannot directly perceive the causes of our ideas but proposed to make a science analyzing those causes, anyway.

If the association of ideas in Priestley's sense could be accepted into psychology, Darwin's kind of materialist association, with its emphasis on bodies and energy, most certainly could not. In The Marriage of Heaven and Hell (1793), William Blake has the devil say a number of Darwinian things, such as:

1. Man has no Body distinct from his Soul for that called Body is a portion of Soul discernd by the five Senses, the chief inlets of Soul in this age;

2. Energy is the only life and is from the Body and Reason is the bound or outward circumference of Energy;

3. Energy is eternal delight.

Blake, the great antinomian, endorsed these ideas and loathed the orthodox thought that “God will torment Man in Eternity for following his Energies.” But few others in those days would go along with such heretical thinking.

Four major implications of the elder Darwin's fluid materialism threatened to undermine established truths. To the extent that one could talk of a soul within the context of Darwin's theory, it was to be found throughout the body, as Blake and some later Romantics often emphasized. This supposition meant first that the soul lacked mental unity, although it may have had physical coherence; and second that even the visceral feelings would have to be treated as equal in importance to rational thought. As far as orthodox thinkers were concerned, physical lust and ideal love were by no means adequately distinguished in Darwin's theory, which upset the entire established ranking of mental states and thus, third, threatened to obliterate the distinction between animals and human beings. Indeed, like his more famous grandson, Erasmus Darwin tended to emphasize the continuity between animals and men (especially in his last work, The Temple of Nature). Fourth, this theory left no separate and distinct sphere for mental activity. All thought is embodied in our feelings and reactions; hence, there seems to be no place for an immortal soul. It was not even possible to fall back on the old position that the brain is merely a physical vehicle for thoughts and feelings within the soul, because that could only be the case if the sole object of thought were mental events, not states of the nerves, as in Darwin's theory.

It is difficult to ascertain how widely disseminated Erasmus Darwin's views were in Europe. In the periodical literature of the Napoleonic era his name is often mentioned, even in French, Italian, and German sources. His biographer, Desmond King-Hele, perhaps goes too far in making Darwin out to be the greatest scientific intellectual in Europe as the nineteenth century dawned—but then again, perhaps not. Who would have been Darwin's rival? It is difficult to think of a serious competitor even among the French. His relative obscurity nowadays should not lead us to ignore the impact of Darwin's fluid materialism, which was, at the very least, one of the major physiological and psychological theories propounded between 1789 and 1815.

I believe that Darwin's influence on subsequent psychology has been overlooked because of the peculiar form it took. After 1815, opponents of Darwin's ideas took precautions not to mention his name or his writings. And, because no mainstream thinkers adopted his views, he was for decades merely the subject of anonymous abuse, at least as far as “official thinking” went. By the middle of the century, however, the ultimate pattern in the reception of Darwin's ideas was already becoming clear: all Darwin's concepts were taken up as true … of the unconscious mind, not the conscious mind. Both his assertions of fact and his speculations could thus be domesticated: if they did not apply to the conscious mind, they did not affect the human soul.

But although I speak here of reception or assimilation of Darwin's alternative psychology, my terminology may be misleading. In general, and especially after 1815, the officialdom of church and state attacked his theory with full force throughout Europe. The battle was hardly one of ideas alone. No one could hold a professorship anywhere in Europe or the United States and defend Erasmus Darwin's psychology. After 1815 the publication of such views became punishable in most European countries by imprisonment. …