As a distinct science, psychology owes its origin to Aristotle, whose “_Peri Psyches_” is, in all probability, the first formal treatise on the subject. Through his father, Nichomachus, who was court physician to Philip of Macedon, he became acquainted, at an early age, with biological lore in the form of such medical botany, anatomy, and physiology as were commonly known in prescientific days. Subsequently, his celebrated pupil, Alexander the Great, placed at his disposal a vast library, together with extensive opportunities for biological research. This enabled the philosopher to criticize and summarize the observations and speculations of his predecessors in the field, and to improve upon them by means of personal reflection and research. In writing his psychology, he was naturally forced to proceed on the basis of the facts discoverable by internal experience (introspection) and unaided external observation. Of such facts as are only accessible by means of instrumentation and systematic experimentation, he could, of course, know nothing, since their exploration awaited the advent of modern mechanical and optical inventions. But the factual foundation of his treatise, though not extensive, was solid, so far as it went, and his selection, analysis, and evaluation of the materials at hand was so accurate and judicious, that the broad outlines of his system have been vindicated by the test of time, and all the results of modern experimental research fit, with surprising facility, into the framework of his generalizations, revision being nowhere necessary save in nonessentials and minor details. Wilhelm Wundt, the Father of Experimental Psychology, pays him the following tribute: “The results of my labors do not square with the materialistic hypothesis, nor do they with the dualism of Plato or Descartes. It is only the animism of Aristotle which, by combining psychology with biology, results as a plausible metaphysical conclusion from Experimental Psychology.” (“Grundzüge der physiologischen Psychologie,” 4te Auflage, II, C. 23, S. 633.) Literally translated, the title of Aristotle’s work signifies a _treatise concerning the soul_. It set a precedent for the scholastic doctors of the thirteenth century, and _de anima_ became with them a technical designation for all works dealing with this theme. In the sixteenth century the selfsame usage was embalmed in the Greek term psychology, which was coined with a view to rendering the elliptic Latin title by means of a single word. Melanchthon is credited with having originated the term, which, in its original use as well as its etymology, denoted a science of the _psyche_ or soul.