This paper offers a reassesment of Simon Stevin's mechanics, by focusing on how Stevin tries to anchor his mathematical demonstrations in the behavior of material instruments. It is shown how his views on the relation between spiegheling (speculation) and daet (practice) are crucial to correctly understand his famous proof of the law of the inclined plane and his experimental test of the Aristotelian law of free fall. The distance separating spiegheling and ">daet is reproduced in that between instruments at rest and instruments in motion, because of Stevin's claim that impediments to motion are "inseperable accidents' of all moving objects.