The subject-matter which had been taken as satisfying the demands of knowledge, as the material with which to frame solutions, became something which set problems. Hot and cold, wet and dry, light and heavy, instead of being self-evident mat- ters with which to explain phenomena, were things to be investi- gated; they were "effects," not causal principles; they set question marks instead of supplying answers. The differences between the earth, the region of the planets, and the heavenly ether, instead of supplying ultimate principles which could be used to mark off and classify things, were something to be explained and to bring under identical principles.