At the centre of it lies what has crystallised in more living shape out of the dry conceptions of the Socratic method—the domain of ideas. Even as Parmenides perceived Being in the eternal All-Existent, accessible to Reason alone, so Plato sees the being of individual things in that which pertains to them in common and as such can be grasped by the Reason. But even as the Eleatic “One” exists even apart from its recognition as an objective being, so these eternal and unchangeable archetypes (ideai) live in and by themselves as objective essences which exist wholly apart from the individual objects which partake of their form. These archetypes, like the Eleatic All-Existent, bear the name of unit (monad), only in Plato’s scheme there are many such monads, and their unchangeableness does not exclude the idea of causation. Thus his “ideas” are the “units” of Parmenides in multiplicity and the “conceptions” of Socrates endued by metaphysics with the breath of life.
To Socrates the idea of Good and of Virtue lay at the heart of his teaching, and thus the preponderance of the idea of Good is confirmed to his pupil, and in its theological elaboration this abstract idea is converted into the Supreme Reason, the first cause of Being, which is identical with the Deity.
As to the Eleatics, the external world was an illusion of the senses, and in any case a thing irrational, so matter and the world of phenomena which occupies the middle place between matter and ideas is hard to grasp, and Plato’s notion of the World-Soul which hovers between the two is as contradictory and obscure as that of the human soul. For with this gifted poet-philosopher there is much that tarries on the threshold of consciousness, and fails to struggle into clear light, a circumstance that harmonises with his own teachings, which find clearness and singleness of purport in the Eternal and Divine alone, obscurity and ambiguity in the intermediate terrestrial sphere of genesis, and utter darkness and inconceivability in the lower sphere of matter and non-existence. These three stages are repeated in his theory of the soul, which from desire rises to courage and ultimately to reason. His ethics and politics, which according to his Hellenic ideas are one and the same, are calculated for three classes of humanity—the iron, the silver, and the golden. The last two, the military and learned classes, are the only ones taken into account in the educational system of his ideal state; for the proletariat there is no need to be concerned, although Antisthenes and his successors regarded this very class as the only one capable of genuine philosophy. But Plato, like the aristocrat he was, has in view an elect type of humanity, exalted by exceptional intelligence above the brute multitude and the solid middle-class element and called by philosophy, i.e., the doctrine of ideas, to the helm of the ideal state.
The teaching of the Sophists had abolished law. Plato likewise knows no law on the lofty level of his ideal state. But the constraint of law seems superfluous where each individual is trained to be the ideal man. Forced by bitter experience to moderate his demands upon human nature and the state towards the end of his life, he sketched in the Laws, a model state on the basis of the old established system of government. But this system, like the metaphysics of his old age, seems, as it were, a desertion of his ideals. All that Plato achieved was the education of a race of pupils in his Academy who far surpassed the common standard of learning and morals, and who, though unable to save the state, yet maintained a high standard of knowledge and an ideal of morality for mankind in the midst of a corrupt society.
The greatest of these Academicians is Aristotle of Stagira, who displayed a versatility and thoroughness of research which appears absolutely incomprehensible in our eyes. Like Plato, he steadfastly held that knowledge is never complete, but that truth is to be found by unremitting persistence in inquiry. This is probably the reason why he gave the world some dialogues adapted to the public taste, and with the help of some of his pupils accumulated and published collections of historico-philological and scientific matter in an unpretentious form; but the systematic lectures in which he propounded to the more advanced followers of his school the results of his speculations and of his wide empirical observation, together with a critical treatment of his predecessors, were never published by him. He worked at these papers his whole life long, and many of the didactic writings which were edited by his pupils after his death, and which are all we possess of the whole body of Aristotle’s works, bear evident traces of gradual growth, correction, and amplification.
In a sketch like the present it is impossible to give so much as a summary of the contents of this admirably arranged encyclopædia, which ranked as the richest storehouse of every kind of empiric and speculative science from the beginning of the Christian era down to modern times. The essential points in which his life-work makes an advance on that of Plato are as follows:
Plato never went so far as to reduce his great discoveries and intuitions in every department of science to a complete and connected whole, being averse, on scientific and ethical grounds alike, from the dogmatic definition inseparable from any systematic treatise. This Aristotle did, dividing the whole body of philosophy under three principal heads (theoretical, practical, and poetical) and distinguishing subdivisions (logic, physics, metaphysics, ethics, and politics, and so forth) within these divisions by strongly marked lines of demarcation and methods rigorously exact. He is a Platonist in all things and feels himself so to be. Even where he displays most independence, as in the development of syllogisms or in biology, it is impossible to overlook his indebtedness to the bold speculations of the master.
If the whole work of Plato’s life and of his scholars between 388 and 348 had been preserved to us, the ultimate connection between Aristotle and the researches of the Academy would probably be even more evident than it is. Nevertheless there is a marked difference between the speculations of these two great philosophers. Plato wholly dissevered the Universal and Essential in things from the Terrestrial and placed it in a heaven beyond the earth.
Aristotle repudiates this transcendentalism all along the line. The Universal cannot exist without the archetype, the essence must be immanent in it. Hence the individual is the only true Substantive, containing Substance and Matter. This opposition of opinion concerning “Universalia” is, as is well known, the starting-point of mediæval Scholasticism (Nominalism, Realism).
The motion of passive substance towards the active form, i.e., the realisation of the Possible, leads up to the idea of development, of genesis (though not, indeed, in the modern sense) on which Plato’s speculations had made shipwreck, and passes over Plato’s rigid Eleatism to join hands with Heraclitus, the philosopher of change, with whom Aristotle sees the ultimate cause of all motion and all things in the Deity, itself as eternal as the world, which “yearns towards It as the bridegroom towards the bride.” Thus soul, too, is the pattern of the body, hence the purpose of its being. The body is but the instrument (organon) of the soul. Thus Aristotle first coins the name and idea of organic being and draws a sharp distinction between these animate creatures (plants, animals, and man) and inanimate nature. In ethics and politics his speculation treads in the footsteps of Plato, save that, in this province of thought also, he mitigates the uncompromising rigourism of the master by his innate bias towards the historically-established and practically-possible, and turns it to more profitable uses. The ethico-political speculations of both are, however, adapted to the aristocratic class at that time dominant in Greece. Alexander, the pupil of Aristotle, conquered the East during his master’s life-time, but the philosopher’s opinion that the newly acquired continent should be governed by other laws than those of Hellas was not practically feasible. His ethics failed him utterly in face of the new political situation thus created.