But it is obvious to every one who nowadays indulges in the profitless pastime of studying their writings that, as a whole, they failed in grasping the essential difference between homology (or " affinity," as they generally termed it) and analogy - though this difference had been fully understood and set forth by Aristotle himself - and, moreover, that in seeking for analogies on which to base their foregone conclusions they were often put to hard shifts.