 There are two major problems that the Aristotelian tradition has faced in the modern period. They are: (1) explaining and justifying the sense in which substantial forms have causal or explanatory power, especially in light of a developed mechanistic chemistry or physics; (2) the difficulty in understanding the relation of substantial form to properties, especially those in terms of which a given substance is defined. These are connected. It is because the substantial form has genuine causal force that it is conceived of by Aristotle as a unified entity in its own right, and not just as a name for the collocation of properties in terms of which it is defined.