 Avicenna's doctrine of secondary intelligibles assigns logic a subject matter whose essential properties the logician studies; this makes logic a science in the Aristotelian sense of the term. But—according to the strictures applying to an Aristotelian science—no science can probe the existence of its subject matter, but rather must take it as given from a higher science (in this case, metaphysics). Yet the Categories shuttles between secondary intelligibles and the primary intelligibles which are the pre-condition for the existence of the secondary intelligibles.