The scientific tool for knowing the world is mathematical physics. Although dispute continues as to whether mathematics has an empirical origin or is purely rational, it is universally acknowledged that the development of mathematics is the work of the human mind and its peculiar activity. Leibniz introduced a profound conception in the pre-established harmony of his monadology. The account of the agreement we are discussing; it is concerned with prediction. This chapter proposes to work out an alternative and complementary explanation of the agreement, to show how the initial choice of the objects of science is determined by the conditions imposed on any possible experience. Solids are the type of recognizable objects. However, individual experience does not supply us with an immediate knowledge of solid bodies but with changing perceptions. Their observations are no longer confined to naming objects: they assign to them, by means of regular measurement procedures, localizing coordinates and observable magnitudes.