At the same time Lucretius is resolved that no false analogy shall obscure the distinction between life and the conditions of life. It is for attempting, as he supposes, to efface this distinction that he so sharply criticises the earlier Greek thinkers. He scoffs at Heracleitus for imagining that all forms of existence can be deduced from the single element of fire. The idea of evolution and transformation seems, under some of its aspects, utterly alien to our poet. His intimacy with the world of living forms had accustomed him to view Nature as a vast assemblage of fixed types which might be broken up and reconstructed, but which by no possibility could pass into one another. Yet this rigid retention of characteristic differences in form permits a certain play and variety of movement, an individual spontaneity for which no law can be prescribed. The _foedera Naturai_, as Prof. Sellar aptly observes, are opposed to the _foedera fati_.[206] And this is just what might be expected from a philosophy based on the contemplation of life. For, while there is no capriciousness at all about the structure of animals, there is apparently a great deal of capriciousness about their actions. On the other hand, the Stoics, who derived their physics in great part from Heracleitus, came nearer than Lucretius to the standpoint of modern science. With them, as with the most advanced thinkers now, it is the _foedera Naturai_—the uniformities of co-existence—which are liable to exception and modification, while the _foedera fati_—the laws of causation—are necessary and absolute. In like manner, Lucretius rejects the theory that living bodies are made up of the four elements, much as he admires its author, Empedocles. It seemed to him a blind confusion of the inorganic with the organic, the complex harmonies of life needing a much more subtle explanation than was afforded by such a crude intermixture of warring principles. If the theory of Anaxagoras fares no better in his hands, it is for the converse reason. He looks on it as an attempt to carry back purely vital phenomena into the inorganic world, to read into the ultimate molecules of matter what no analysis can make them yield—that is, something with properties like those of the tissues out of which animal bodies are composed. Thus, while the atomic theory enables Lucretius to account for the dependent and perishable nature of life, the same theory enables him to bring out by contrast its positive and distinguishing characteristics. The bulk, the flexibility, the complexity, and the sensibility of animal bodies are opposed to the extreme minuteness, the absolute hardness, the simplicity, and the unconsciousness of the primordial substances which build them up.