Following the preface, Part 4 of the Ethics opens with eight definitions and an axiom. It begins with these: “4d1. By good [bonum] I shall understand what we certainly know to be useful to us. / 4d2. By evil [malum], however, I shall understand what we certainly know prevents us from being masters of some good.” / Goodness, according to 4d1, is the property of being useful, or advantageous. 4d2 says in effect that evil is a matter of having disutility, or being disadvantageous. The value of something is determined by how it well it serves someone. A thing's disvalue is determined by the severity of its disservice to someone. Thus 4d1 makes clear that, for Spinoza, goodness, or value, is about being useful. Things are considerably less clear when it comes to Spinoza's theory of the good, his account of our ultimate end or summum bonum. Granted that something is good in case it is useful, the question is, useful for what? What, if anything, is the ultimate end or purpose by which to measure the utility of things?