 Eventually, the demand that premises be “better known” leads back to the first principles of the science, which are self-evident (per se nota) in the sense that they are known to be true simply in virtue of their terms (Comm. Post. Analytics I.7.8). Nothing could be better known than such a principle, but because these principles are self-evident there is also no need for any further demonstration. Each science has its own starting points—its own first principles—which are not susceptible to demonstration within that science. But not all such first principles are strictly self-evident. Within physics, for instance, it is a first principle that there is motion, but that is neither absolutely necessary nor strictly self-evident. Accordingly, again following Aristotle, Aquinas orders the various sciences according to relations of “subalternation”, such that biology is subalternate to physics, and physics to metaphysics. In general, “the principles of a lower science are proved by the principles of a higher science” (Comm. Post. Analytics I.17.5). Among the natural speculative sciences—that is, setting theology and ethics aside—the preeminent science is metaphysics, or first philosophy, “from which all the other sciences follow, taking their principles from it” (Comm. Boethius De trin. 5.1c). (On the structure of the sciences see Maurer 1986; Wippel 1995a; Dougherty 2004.)