 The extant Principles offer metaphysics in Part I; the general principles of physics, in the form of his matter theory and laws of motion, are presented in Part II, as following from the metaphysics; Part III concerns astronomical phenomena; and Part IV covers the formation of the earth and seeks to explain the properties of minerals, metals, magnets, fire, and the like, to which are appended discussions of how the senses operate and a final discussion of methodological issues in natural philosophy. His intent had been also to explain in depth the origins of plants and animals, human physiology, mind–body union and interaction, and the function of the senses. In the end, he had to abandon the discussion of plants and animals (Princ. IV.188), but he included some discussion of mind–body union in his  abbreviated account of the senses.