In this argument Aristotle undoubtedly expressed a profound truth, but it may perhaps be admitted that he rather failed to appreciate fully the difficulty which the Platonic doctrine was designed to meet--that, namely, of providing some sort of common nexus or unifying principle by which the validity of Knowledge could be maintained. For he had no certain means of showing that the potent energy of Nature was unitary and homogeneous. He is frequently described as a sensationalist, but such a view is certainly incorrect. This, however, may be admitted--that he sought the essentials of Reality not in the Mind but in the Object. It may be fairly claimed that to this extent he occupied common ground with the sensationalists, in that he was an adherent of the _tabula rasa_ view of the Mind, expressed in the maxim:-- _Nihil est in intellectu quod non fuit in sensu._ * * * * * Plato and Aristotle may be taken as typical of the two principal intellectual tendencies which have characterised all subsequent speculation--the Platonist, he who finds in the constitution of the Mind the eternal principles or at least the types of the eternal principles of Reality; the Aristotelian, he for whom these seem to reside in the object and, in the act of Cognition, are merely impressed upon, transferred to, presented to, or otherwise introduced into or apprehended by the Mind. The Aristotelian view of Nature as an energetic process failed to impress itself upon his successors. Greek Philosophy soon after Aristotle's death decayed or was deprived of its early vigour, and the doctrine which survived the wreck was essentially derived, however imperfectly, from the Platonic theory. Throughout the first fifteen hundred years of the Christian era this doctrine undoubtedly dominated the course of speculation--a speculation of which much is now forgotten and almost as much was certainly barren and unfruitful, but of which we would entertain a very mistaken notion if we were to imagine that it was not often pursued with great subtlety and acumen. One natural result of the fact that such a principle dominated human thought was the prevalence of a belief that the explanation of Nature and natural processes could be derived from the cognitive faculty itself. Our cognition of our immediate surroundings was doubtless continuously corrected by immediate practical tests. But the science of a more extended view of Nature was vitiated by this false principle and in consequence for many centuries our whole Knowledge of Nature remained unprogressive and unfruitful. _Causa æquat effectum_, Nature abhors a vacuum, are examples of the maxims derived or supposed to be derived from the necessities of our Reason, and by the aid of which it was vainly hoped to attain a knowledge of Nature and natural laws. The principle was in itself unsound. The necessary laws of our rational faculty could discover to us only the essentials of that faculty itself.