He was scarcely less ambitious than his pupil Alexander, and his works embrace nearly the whole range of human knowledge as it existed in his day. He was the inventor of the syllogistic mode of reasoning, the principles of which he lays down in his work on logic. In his books on rhetoric, he has investigated the principles of eloquence with great accuracy and precision, insomuch that they form the basis of all that has since been written on the subject. His work on poetics, or rather the fragment which has come down to us under that name, although almost entirely confined to the consideration of the drama, contains principles applicable to poetical composition in general, and is equally distinguished for precision and depth of thought. Those on ethics and politics are also remarkable productions; and although the former has been effectually superseded by a more perfect system, the latter contains much that is interesting even at the present day. In his metaphysics, he expounds the doctrine of Being abstracted from Matter, and speaks of a First Mover,--the life and intellect of the universe, eternal and immutable, but neither omnipresent nor omnipotent. When treating of physics, he does not in general lay down rules _a priori_, but deduces them from the observation and comparison of facts. This being the case, we might expect that such of his writings as relate to natural history should contain much truth. He holds that all terrestrial bodies are composed of four elements,--earth, water, air, and fire. Earth and water are heavy, because they tend towards the earth's centre; while air and fire, which tend upwards, are light. Besides these four elements, he has admitted a fifth, of which the celestial objects were composed, and whose motion is always circular. He supposed that there is above the air, under the concave part of the moon, a sphere of fire to which all the flames ascend, as the brooks and rivers flow into the ocean. He maintains that matter is infinitely divisible; that the universe is full, and that there is no vacuum in nature; that the world is eternal; that the sun, which has always revolved as it does at present, will for ever continue to do so; and finally, that the generations of men succeed each other without having had a beginning or foreseeing an end. He alleges that the heavens are incapable of decay; and that although sublunar things are subject to corruption, their parts nevertheless do not perish; that they only change place; that from the remains of one thing another is made; and that thus the mass of the world always remains entire. He holds that the earth is in the centre of the world; and that the First Being makes the skies revolve round the earth, by intelligences which are continually occupied with these motions.