e passage from ordinary perception to scientific knowledge did not therefore demand the introduction of actual, overt and observed changes into the material of sense perception. Modern science, with its changes in the subject-matter of direct perception effected by the use of apparatus, gets away not from observed material as such, but from the qualitative characteristics of things as they are originally and "naturally" observed. It may thus be fairly asserted that the "categories" of Greek description and explanation of natural phenomena were esthetic in character jfor perception of the esthetic sort is interested in things in their immediate qualitative traits. The logical features they depended upon to confer scientific form upon the material of observation were harmony, proportion or measure, symmetry: these constitute the "logos" that renders phenomena capable of report in rational discourse. In virtue of these properties, superimposed upon phenomena but thought to be elicited from them, natural objects are knowable. Thus the Greeks employed thinking not as ameans of changing given objects of observation so as to get at the conditions and effects of their occurrence, but to impose upon them certain