  Ockham thought that the causal power of a most specific natural kind (infima species) can be known through the observation of a single instance. So the curative power of an herb, or the ability of heat to produce heat in something adjacent to it, can be concluded by reason from a single instance. The principle at work here states that everything in a given most specific natural kind which acts produces effects of the same kind. He thus seems to refer to a version of the uniformity of nature. He is perfectly clear that the real definition of a simple substance, since it does not signify anything outside the substance, cannot tell us what its effects are. Here we find the roots of Early Modern Empiricism.