Immanuel Kant copernican revolution in philosophy had consisted in the idea that the objects of our cognition must in some sense be phenomena that “conform to” certain conceptual rules of synthesis and spatio-temporal forms of sensibility that are contributed by the mind a priori, independently of experience. In relation to the classical debate, Wilfrid Sellars at mid-century sees the correct account as lying in a Kantian middle ground between logical empiricism and metaphysical or rationalist realism. On Sellars’s view, lawlike statements and assertions of causal connection make explicit the corresponding implicitly endorsed material rules of inference. Bertrand Russell in various respects throughout his career defended rather than rejected Kant’s claim that there are certain synthetic a priori principles. The synthetic a priori was particularly prominent in Russell’s 1912 introductory work, The Problems of Philosophy, and it included at that time various principles of logic and mathematics.