THREE: His utterances on this subject [the existence of an external world] are perhaps chiefly to be found in the third book of his treatise On the Soul, beginning with the fourth chapter. On turning to them we see that he never separates existence from knowledge. A thing in actual existence, he says, is identical with the knowledge of that thing. Again, The possible existence of a thing is identical with the possibility in us of perceiving or knowing it. Thus, until a thing is perceived or known, it can only be said to have a potential or possible existence. And from this a doctrine very similar to that of Ferrier might be deduced, that nothing exists except plus me,that is to say, in relation to some mind perceiving it. (Aristotle, p. 165.)In this respect Stoicism betrays its descent from the encyclopaedic lectures of the earlier Sophists, particularly Hippias. While professedly subordinating every other study to the art of virtuous living, its expositors seem to have either put a very wide interpretation on virtue, or else to have raised its foundation to a most unnecessary height. They protested against Aristotles glorification of knowledge as the supreme end, and declared its exclusive pursuit to be merely a more10 refined form of self-indulgence;20 but, being Greeks, they shared the speculative passion with him, and seized on any pretext that enabled them to gratify it. And this inquisitiveness was apparently much stronger in Asiatic Hellas, whence the Stoics were almost entirely recruited, than in the old country, where centuries of intellectual activity had issued in a scepticism from which their fresher minds revolted.21 It is mentioned by Zeller as a proof of exhaustion and comparative indifference to such enquiries, that the Stoics should have fallen back on the Heracleitean philosophy for their physics.22 But all the ideas respecting the constitution of Nature that were then possible had already been put forward. The Greek capacity for discovery was perhaps greater in the third century than at any former time; but from the very progress of science it was necessarily confined to specialists, such as Aristarchus of Samos or Archimedes. And if the Stoics made no original contributions to physical science, they at least accepted what seemed at that time to be its established results; here, as in other respects, offering a marked contrast to the Epicurean school. If a Cleanthes assailed the heliocentric hypothesis of Aristarchus on religious grounds, he was treading in the footsteps of Aristotle. It is far more important that he or his successors should have taught the true theory of the earths shape, of the moons phases, of eclipses, and of the relative size and distance of the heavenly bodies.23 On this last subject, indeed, one of the later Stoics, Posidonius, arrived at or accepted conclusions which, although falling far short of the reality, approximated to it in a very remarkable manner, when we consider what imperfect means of measurement the Greek astronomers had at their disposition.24
