This chapter argues that Hegel’s aim in his philosophy of nature is not to compete with natural science but to show that there is reason in nature – reason that science cannot see but that works through the causal processes discovered by science. It considers first the transition from Hegel’s logic to his philosophy of nature and argues that the latter continues the project of the former, starting with reason, or the “absolute idea”, as nature, as sheer externality. It then argues that Hegel derives nature’s categories logically – a priori – from the idea-as-externality, and subsequently matches them with empirical phenomena (rather than constructing categories to fit the latter). It provides an abridged account of Hegel’s physics in order to show how the categories of physical (as opposed to mechanical or organic) nature are derived from one another and how they are embodied in physical phenomena, such as sound, heat, and magnetism. It then concludes by arguing that, contrary to appearances, Hegel’s conception of light complements, and is not simply at odds with, that presented by quantum physics.