 In order to understand the Stoic claims about the relation of virtue to happiness, we can begin with virtue. Chrysippus says that virtue is a craft (technê) having to do with the things of life (SVF II 909). In other texts, we learn that the things of life include impulse (hormê). Each animal has an impulse for self-preservation; it has an awareness of its constitution and strives to preserve its integrity. There is also a natural impulse to care for offspring. Humans, then, are naturally inclined toward preserving life, health, and children. But then grown-up humans also do these things under the guidance of reason; reason in the adult human case is the craftsman of impulse (DL VII 85–6). This latter phrase is significant because it implies that just following natural impulse is not enough. In fact, it is not even possible for an adult human, whose nature is such as to do everything they do by reason, even to follow the sort of natural impulses that animals and immature humans do in their actions. In order to lead a virtuous life, reason must shape our impulses and guide their expression in action.