  However, what is more interesting for us are the alternative causal accounts that Buridan and Oresme adopted: they both said that projectiles move violently because of a form inherent in them, which led them to move in a non-natural direction, and which naturally decayed. This form was known as ‘impetus’, and was a common theme in the philosophy of the thirteenth and fourteenth centuries; some version of an impetus theory goes back to the early thirteenth century (Wood 1992). There was, especially in the fourteenth century, a considerable amount of quantitative work on impetus which attempted to establish such things as the law according to which impetus decayed (Weisheipl 1982, pp. 535ff.).