From beginning to end, the Nicomachean Ethics is a work of practical science. While few deny this, its implications remain unclear. I argue that practical science should not rely on any theoretical principles. This position has been insufficiently appreciated and defended. I acknowledge that the practical treatises are largely consistent with the theoretical books, and even that they might not have been written had Aristotle not worked out his theoretical positions. Nonetheless, they do not depend upon them or ever appeal to them explicitly for any sorts of principles. This does not preclude his using points from the logical works within practical science, as in any of the theoretical sciences, but theoretical conclusions and premises are out of bounds. This is important for understanding the argumentation of this treatise itself, for appreciating Aristotle’s commitment to practical science as science, and for defending the ethical writings from claims that they are outmoded based on superseded theoretical positions. After elucidating the first book of the Nicomachean Ethics, I turn to later sections that might be supposed dependent upon theoretical conceptions.