“ Thus if we observe the heavenly bodies individually we can state merely that they have been observed to have certain motions through the sky, that they are luminous, and the like. If, however, we compare them one with another, we discover that, whereas all partake in the general movement of the heavens, some have a movement of their own. Thus we arrive at a system of classification according to motion, by which fixed stars are differentiated from planets. A further classification according to other criteria gives us stars of the first magnitude and stars of the second magnitude, and so forth. ”

“ Of the systems of classification adopted in physical science, only one requires treatment here, namely, the classification of the sciences as a whole, a problem which has from the time of Aristotle attracted considerable attention. ”

“ In Greece at the dawn of systematic thought the physical sciences were few in number; none the less philosophers were not agreed as to their true relation. The Platonic school adopted a triple classification, physics, ethics and dialectics; Aristotle’s system was more complicated, nor do we know precisely how he subdivided his three main classes, theoretical, practical and poetical (i.e. technical, having to do with ποίησις, creative) . The second class covered ethics and politics, the latter of which was often regarded by Aristotle as including ethics ”