 To illustrate the Stoic doctrine of the internal source of animal motion Chrysippus uses the analogy of a stone cylinder rolling down a slope. The cylinder’s rolling down the slope may require an initial shove. But that initial shove only triggers the motion, and the primary cause of the motion is the cylinder’s own shape and “rollability”. Likewise, human action may require an initial impression as its trigger, but how a person acts depends on the person themself, on the sort of person they are and how they respond to their impressions (Cicero, Fat. 39–43, LS 62C).