For early thinkers, untrained in the methods and unaware of the limits of thought, even for the great and free thinkers of Greece, a captivating analogy was irresistible[26]; while inventing schemes of thought they believed themselves to be describing the processes of nature. Moreover it has been the temptation of philosophers of all times, and even of Harvey himself than whom none had put better the conditions of scientific method, to suppose that by means of abstraction kinds may be apprehended; that thus they may get nearer to the inmost core of things; that by purging away the characters of individuals they may detect the essence and cause of individuation (σπερματικὸς λόγος): not perceiving indeed that the content of notions is, as Abélard had pointed out plainly, in inverse proportion to their universality. Like Sidney’s hooded dove, the blinder they were the higher they strove[27]. For example: from a lump of silver a medal is struck; from many lumps of silver many medals are struck, each different from the other: let us eliminate as accidents the notions of silver, of the blow of a hammer, even of particular features of the devices, and we shall reach the idea of an agent with a type or seal, or of such an agent with many seals, or ideas, who may thus individualise indifferent matter; or, to penetrate deeper into abstraction, who may transfer forms of his own activity to motionless stuff. It is my part to-day to show that before motionless stuff—before the problem of the “primum mobile”—even Harvey himself stood helpless; helpless yet fascinated by the indulgence of invention when, in the _De motu cordis_, or the _De generatione_, he permitted himself to carry contemplation beyond the sphere of his admirable experiments. “Natural, vital and animal spirits” indeed he would have none of; saying well that he should want as many spirits as functions, and that to introduce such agents as artificers of tissues is to go beyond experience: yet in his need of a motor for his machine he was not able to divest himself of the language nor even of the philosophy of his day; he referred the cause of the motion of the blood, and therefore of the heart, to innate heat[28]. In his day he could not but regard rest and motion as different things; and motion as a super-added quality. In denying the older opinion[29] that the heart is the source of motion, of perfection[30] and of heat, he put the difficulty but one stage back; and, when in the treatise on Generation he propounded his transcendental notion of the impregnation of the female by the conception of a “general immaterial idea,” we find in him realism still very much alive indeed. Had Harvey been content with innate heat he would have done well enough; but the innate heat of the blood, as he explains it, is not fire nor derived from fire; nor is the blood occupied by a spirit, but is a spirit: it is also “celestial in nature, the soul, that which answers to the essence of the stars ...