We thus oppose justice and injustice, because a habit or trained faculty differs in this respect both from a science and a faculty or power. I mean that whereas both of a pair of opposites come under the same science or power, a habit which produces a certain result does not also produce the opposite result; e.g. health produces healthy manifestations only, and not unhealthy; for we say a man has a healthy gait when he walks like a man in health.

[Not that the two opposites are unconnected.] In the first place, a habit is often known by the opposite habit, and often by its causes and results: if we know what good condition is, we can learn from that what bad condition is; and, again, from that which conduces to good condition we can infer what good condition itself is, and conversely from the latter can infer the former. For instance, if good condition be firmness of flesh, it follows that bad condition is flabbiness of flesh, and that what tends to produce firmness of flesh conduces to good condition.