  The stipulation that the premises of a scientific demonstration must be indemonstrable, that is, self-evident, leads to a prima facie difficulty. This conception of scientific knowledge is intended to encompass not only conceptual propositions, but also propositions about the real natures or essences of substances, which is to say propositions about the world. As with any other demonstration, a demonstration in natural philosophy must have premises that are self-evident, since otherwise a regress would ensue. Yet it is not clear, at least from a contemporary perspective, how the premises can be self-evident, since they must be experience-based.