The paper deals with some of the innovations that Thomas Aquinas introduced into Aristotle’s doctrine of the One. On the basis of Aquinas’ definition of the One as undivided being (ens indivisum), the author attemps to demonstrate three such innovations. Firstly, the nature of the negation which is included in the concept of the One is negation in the sense of privation. Secondly, with a view to the kind of division being negated by the One, the formal and material division is distinguished in order to show Aquinas’ sharp distinction between the One as equivalent to being and the One which is the principle of a number. Finally Aquinas’ concept of the equivalence of being and the One is explained as well as how this equivalence is ontologically grounded in being (esse) as actuality.