 The main sources of evidence for Spinoza's thought concerning physical nature are his Principles of Cartesian Philosophy, the first half of his magnum opus the Ethics, and a number of important letters from his correspondence. The PCP contains by far the most focused and detailed of Spinoza's writing on physics proper, but is officially billed as an exposition of Descartes. The first half of the Ethics presents a physical theory only insofar as Spinoza finds this necessary to serve his goals in that work to explicate the nature of God and the natures and origins of the human mind and its affects. Discussion of the physical in the correspondence, though often illuminating, is typically directed at providing either clarification or defense of his views in response to queries and objections, or at discussing contemporary views and developments in experimental and theoretical science in order to show that they are either mistaken or consistent with his own views.