 There is a second line of evidence, quite independent of these programmatic statements, which leads to the same conclusion. The topics covered in PA I take the form of specifications of the central topics of the Posterior Analytics. These specifications are required because animals are [a] complex unities of matter (body) and form (soul); [b] arise by a complex process of development; [c] the end—that for the sake of which the development occurs— is both causally and definitionally prior to that process; [d] a distinctive kind of necessity, conditional necessity, is operative; and [e] a special method of multi-differentiae division is required. Such a discussion is required by the fact that although the Posterior Analytics intends its epistemic standards to be applicable to natural science – as is clear from the many examples drawn from natural science in book II – it provides no details as to how this application is to be accomplished.