The theory by which he attempted to explain the gra dual decay of the system was ingenious, but extremely fanciful. lie imagined that prime matter was first con verted into bodies or triangular shapes; that of these bodies the elements were constituted, and assumed re gular geometrical figures,—fire being a pyramid, the earth a cube, the air an octahedron, and water an icoso hedron: that, as the body was composed of these ele ments, their angles would in course of time be blunted, and thus gradually be rendered unable to retain their hold, till the fabric would dissolve, and the soul, like a ship loosed from her anchor, be left to drive at plea sure. Another phenomenon, no less curious, engaged the attention of this speculative philosopher. It had been observed, that the forms of bodies, though almost infi nitely various, were vet strictly limited in their number, and that every species was uniformly distinguished by the same permanent characters. To account for this singular fact, Plato supposed, that, besides matter and mind, there were certain forms which lie calls /hal ; a kind of moulds in which matter is cast, and which regu larly produce the same number and variety of species in the animal, mineral, and vegetable kingdoms.
The difference of reason, instinct, and passion, indu ced Plato to believe, that, instead of one vital principle, there must be three, one for each of the great cavities of the human body; the head being the scat of the ra tional principle; the heart of the courageous; and the sensual grovelling principle residing in the lower belly, to which the liver is a kind of mirror, reflecting certain spectres and phantoms. in these principles may be easily discerned a modification of the d of 11 ippo crates ; and his :vvay.cic seem to have sprung from the old superstition, which assigned to a tutelary genius the charge of each of the bodily organs. Thc L.v0f4E16 were converted by the Latins into fit( rata tea and Virt8 ; and hence the fitcraiies of our metaphysicians; the via gene t rix, via concoc t rex, and via med x , and other sires of our physiologists, known in English by the name of The next philosopher who claims our attention is Aristotle, the pupil of Plato; but in every respect an exact counterpart to his master. lle possessed not that glowing eloquence, nor that sublimity ol conception, which distinguished the illustrious founder of the Aca demy ; but he surpassed him far in perspicuity, pre cision, and arrangement; in that cool and determined industry, which alone could guide him steadily in the path of discovery; in a vast comprehension of mind, which enabled hint to grasp the whole science of his time ; and in an acuteness of discrimination, which led him to generalize his ideas, and to refer every object of knowledge to its proper class. With such qualifications., it is not to be wondered that he should gain an absolute control over the literary world. Ile wrote on almost every branch of science; and his writings displayed such skill and force of reasoning, that they acquired the authority' of an oracle. Ile has fallen, indeed, into many
errors, but these must in general be ascribed to the dis ad•antages of his situation ; nor is it to be doubted, that. had he lived in modem times, he would have risen to unrivalled eminence, and perhaps have acquired as un liinited authority as he exercised over his own and succeeding ages.
At the request of his pupil, Alexander the Great, he undertook to write the natural history of animals. For this purpose Alexander supplied him with 800 talents, and employed men in every part of his extensive em pire, to collect animals of all descriptions for his use. The philosopher, thus assisted, was enabled to arrange them into classes; to mark their various forms, dispo sitions, and habits, and the peculiar motions, and mode of generating, by which each class is distinguished. His observations ai'e exceedingly valuable ; and although his distinctions be not always adopted, many of them, at least, are retained ; both the paths and objects of inquiry are clearly' marked out; and his history of animals has not only tended to inspire an enthusiasm for such hives tigations, but to abridge their toil, and has furnished many important hints to succeeding naturalists.
He likewise inquired into the internal structure of animals; wrote a treatise on anatomy, and gave an ana tomical nomenclature, both of which have unfortunately been lost. He examined the different organs with con siderable minuteness; observed their form, situation, and structure ; and has given a number of names and distinctions, which are still found of considerable use. it was owing to the state of science in his time, that, notwithstanding his acquaintance with the animal struc ture, his physiology is as imperfect as that of Hippo crates. He supposed the arteries to be organs of mo tion; and the flesh which moves an organ of sensation. A ligament lie terms a nerve; and if he was really ac quainted with the organs which we distinguish by that name, he had not the most distant idea of their func tions. The brain he fancied a mere mass of water and earth, designed to exhale vapours, and to assist the lungs and the arteries (which he supposed to be air vessels) in regulating the degrees of animal tempera ture. He believed that the heart is not only the limn tain of the blood, and the origin of the veins, but the origin likewise of the ligaments and tendons; the organ of motion, sensation, and nutrition; the seat of the soul, of the passions, and of the vital flame. To account for respiration, lie supposed that the heat expanded the air contained in the lungs, and that the external air rushed to prevent a vacuum. In his notion of digestion he ex actly coincided with Hippocrates; imagining that the stomach is a passive organ, in which the aliments were boiled, stewed, or concocted ; but that, in this operation, it is assisted by the heat of the liver, the spleen, and the viscera in the neighbourhood ; that the liver, the spleen, and the kidneys, however, are also intended to serve as cushions to the arteries and the veins.