Post-Cartesian Occasionalism argues that the power of causing an effect depends on knowledge of the means by which the effect is produced. The argument is used to deny finite beings the power to act. Arnold Geulincx expresses this thesis in the principle Quod nescis quomodo fiat id non facis. Here, my purpose is to show that: 1. The philosophical problem that is at the origin of the principle Quod nescis quomodo fiat id non facis originates in Galen’s De foetuum formatione, (...) a work translated into Latin only in 1535. 2. Important works of early modern philosophy, such as Campanella’s Del senso delle cose e della magia, discuss Galen’s text. 3. Due to their rejection of teleology, Descartes’ physics and metaphysics are completely foreign to the Quod nescis principle. Comparing the Cartesian theory of animal-machines with the theory of animal behavior of Pierre Chanet, a philosopher who adopts the principle, confirms this claim. (shrink)