The difficult phrase ὅ ποτε ὄν ἐστι (hereafter ‘OPO’), which occurs in key passages in Aristotle’s discussions of blood and of time, has long vexed interpreters of Aristotle. This paper proposes a new interpretation of OPO, which resolves some textual and interpretative problems about Aristotle’s theories of blood and of time. My interpretation will also shed light on more general issues in Aristotle’s metaphysics. In the passages I will discuss, Aristotle takes both blood and time to be examples of his peculiar ‘coupled entities’. He then uses OPO to provide explanations which differ from what we might call his ‘standard’ metaphysical explanations.
On the ‘standard’ approach, Aristotle explains derivative entities—non-substances—by describing their relationship to substances. By contrast to this standard form of explanation, when Aristotle uses OPO he explains blood and time by describing their relationship to non-substances. This paper thus identifies a new species of metaphysical dependence in Aristotle. In addition, it provides detailed examination of evidence concerning whether Aristotle himself used coupled entities in his own physical and metaphysical theories