Philosophy starts from some deep and wide way of responding to the difficulties life presents, but it grows only when material is at hand for making this practical response conscious, articulate and communicable. Accompanying the economic, political and ecclesiastical changes which were alluded to in an earlier lecture, was a scientific revolution enormous in scope and leaving unchanged almost no detail of belief about nature, physical and human. In part this scientific transformation was produced by just the change in practical attitude and temper. But as it progressed, it furnished that change an appropriate vocabulary, congenial to its needs, and made it articulate. The advance of science in its larger generalizations and in its specific detail of fact supplied precisely that intellectual equipment of ideas and concrete fact that was needed in order to formulate, precipitate, communicate and propagate the new disposition. Today, accordingly, we shall deal with those contrasting conceptions of the structure and constitution of Nature, which when they are accepted on the authority of science (alleged or real), form the intellectual framework of philosophy. _Contrasting_ conceptions of ancient and modern science have been selected. For I see no way in which the truly philosophic import of the picture of the world painted by modern science can be appreciated except to exhibit it in contrast with that earlier picture which gave classic metaphysics its intellectual foundation and confirmation. The world in which philosophers once put their trust was a closed world, a world consisting internally of a limited number of fixed forms, and having definite boundaries externally. The world of modern science is an open world, a world varying indefinitely without the possibility of assignable limit in its internal make-up, a world stretching beyond any assignable bounds externally. Again, the world in which even the most intelligent men of olden times thought they lived was a fixed world, a realm where changes went on only within immutable limits of rest and permanence, and a world where the fixed and unmoving was, as we have already noted, higher in quality and authority than the moving and altering. And in the third place, the world which men once saw with their eyes, portrayed in their imaginations and repeated in their plans of conduct, was a world of a limited number of classes, kinds, forms, distinct in quality (as kinds and species must be distinct) and arranged in a graded order of superiority and inferiority.