The Stoics’ way of presenting principles – the active and the passive – is ambiguous because they say that principles are two while also suggesting that they are inseparable and thus interdependent. This ambiguity cannot be resolved in favour of one or the other side of the dilemma, as is shown by analysis of two possible models of the relations among principles – a causal and a categories-based model. This ambiguity is rather a necessary consequence of the Stoic view of (...) principles and should be compared to the ambiguity of Plato’s concept of “principles” in the Timaeus. Plato’s Receptacle is in a similar relation to other constitutive elements of his cosmogonical account as are the two Stoic principles, each to the other. In particular, the relation between the Receptacle and qualities in it is to be seen as a systematic parallel to the relation of the Stoic principles (and probably also its historical model). The concluding claim is that Plato and the Stoics advance a similar kind of dualism which should be called such, despite its ambiguity. The ambiguity in both systems is due to the need to see the principles in such a relation as would reveal their dependence, and thus secure the basic unity of the cosmos. (shrink)