Philosophy Discussion Forums

How does Descartes make the study of physics a priori? It seems to me that he is basing it upon his claim that the true essence of things are in the understanding and based upon that discussing the concept of extension of bodies. Did I understand it correctly? and if I did, still how can we attribute anything to these bodies without observing what properties they actually have?

Decartes is said to be a'rationalist' which means that he believes it possible for a person to stand,as it were, outside of the world and describe it objectively. I understand the Einstein showed that empirical findings are relative to the position of the observer.