  Already in Aristotle, the assertion of the transgeneric character of the one, that runs through the categories in the same way as the concept of ‘being’, and hence is convertible with it, is linked with the designation of ‘indivisibility’ as the proper meaning of unity, and is connected with the categorical one: it is “the first measure of a kind, and above all of quantity; for it is from this that it has been extended to the other categories.” (Aristotle, Metaph. X, c. 1, 1052 b 18–19.) In his most influential, yet highly controversial elaboration on the concept of unity, Ibn Sina agrees with Aristotle that ‘one’ is co-extensional with ‘being’ and differs from it in its concept; he adds, however, that just like being, unity is not part of the substance of a thing, and hence accidental in the sense of extra-essential, a feature that he combines with its being accidental in the sense of being the principle of number in the category of quantity. Already Ibn Rushd criticized Ibn Sina for this confusion of the metaphysical ‘one’ with the mathematical ‘one’. The different reactions in the Latin medieval doctrines of the transcendentals reflect the differences in their internal systematics. Aquinas accuses Ibn Sina of being deceived by the equivocity of the one, confusing the one that is convertible with being and the one that is the principle of number. The point for Aquinas is that the one that is convertible with being adds only something conceptual to being, i.e. the negation of division; whereas the one that is the principle of number adds something real, i.e. the relation to a measure, as a consequence of which the extension of the concept of being is narrowed down to the category of quantity. Duns Scotus, however, agrees with the position that he ascribes to Ibn Sina, namely that being and the one are not essentially convertible, and therewith expresses the major divergence of his doctrine of the transcendentals, in which the transcendentals do not add merely something conceptual to ‘being’, but something really distinct, in the sense in which a property is really distinct from the subject. The argument, however, that Duns Scotus provides for his position, namely the observation that if ‘being’ and ‘one’ were essentially convertible, then multitude would be devoid of reality, is loyal to a major inspiration of Aquinas’ reflection on transcendental unity. Aquinas is also concerned to grant a positive meaning to multitude and even accords a transcendental dimension to multitude.