 Therefore, only general experience is useful to science, but Avempace ranks experience not as high in certitude as the first intelligible but second to it, although he makes it essential part of medicine, insofar as it can yield universal propositions. Avempace occupied himself again with the issue of the classification of sciences in his introduction to the Book of Animals (IB-BA). He placed the science of animals within natural philosophy, which falls again within theoretical philosophy.