  Aristotle reports (Posterior Analytics. i.5, cf. Metaphysics xiii.2) that whereas mathematicians proved theorems such as a : b = c : d =&gt; a : c = b : d (alternando) separately for number, lines, planes, and solids, now there is one single general or universal proof for all (see Section 3). The discovery of universal proofs is usually associated with Eudoxus' theory of proportion. For Aristotle this creates a problem since a science concerns a genus or kind, but also there seems to be no kind comprising number and magnitude. Some scholars have proposed that a universal science of ‘posology’ (a science of quantity) takes the whole category of quantity as its subject.