 Thus, if all birds have beaks, feathers, wings of a sort, are bipedal, etc. , the biologist needs to perform the proper sort of division on all of these in order to grasp the various sub-kinds of bird. This will not, of course, give us demonstrative knowledge; but it will help us to grasp the co-extensively correlated features, at each level of specificity, that are the proper things about which to seek demonstrative knowledge.