The impact rules are theoretical devices conceived by Descartes to determine how bodies increase, decrease or veer their movements, due to the strike with others. Such rules are visibly formulated in The Principles of Philosophy in 1644. The focus point of this article is to analyze how the mentioned rules are established through the application of the third law of nature. The context in which this analysis is framed is represented by the arguments around the interpretation of the relations between physics and metaphysics in the Cartesian work.