For example, the wood while substance is power, therefore can assume diverse forms. The according to estagirita form is the act, therefore, as well as the power she is conditional for the act, equally the substance it is for the performance of the form. In this relation between form and substance in the composition of the substance, the act prevails on the power, in how much that in the form, it has the exclusiveness of the act, and in the substance, it has a mesclagem between power and act. The substance, as well as harnesses is responsible for the finitude of the beings, that is, they are limiter of the form and the act. But the beings incorporeal, that they are exclusively formal, they are perfect, immovable and perpetual, that is, they are pure acts. Consideraes final Therefore, Aristotle influenced for the effective problem in the philosophy, on the question of devir, resultant of the fidgets of the daily pay-socrticos, mainly Parmnides and Heraclitus, establishes the concept of act and power and the relation between itself in the degree of importance of the value, the causalidade and the composition and the relation of them with the composition of the substance.