This dissertation consists of three chapters, each developing one of the three elements of the subtitle. The first, "Aristotle against Eleatic monism", analyses Aristotle's refutation of Eleatic monism in Physics 1.2-3 in parallel with Plato's parricide of Parmenides (Sophist 242b6-5e5) and then compares the Physics' refutation with Aristotle's treatment of the Eleatics in Metaphysics A.3-5. It concludes that from this background Aristotle draws a necessary requirement for his own metaphysical investigation, namely that we regard being as not univocal.

The second chapter, "Aristotle's parricide of Plato", completes the parallelism between the Sophist and the Metaphysics by examining Aristotle's formulation and discussion of the eleventh aporia in Metaphysics Beta: "Are being (to on) and the one (to hen) the substance(s) of all things or are they just attributes of some other underlying thing?". It focuses on a particular characteristic of aporia 11, namely its allocation of the two alternatives of the dilemma to Plato and the Pythagoreans on the one hand and to Empedocles and the physicists on the other. Crucially, it shows that Aristotle not only borrows from the Sophist the arguments which he levels against the pluralists, but also turns the Platonic critique of Parmenides against Plato himself.

The third chapter, "Aristotle's last word on aporia 11", explains how Aristotle resolves this metaphysical dilemma in Metaphysics I.2. It shows that he does not return to it until he has developed new weapons against Plato and against the Pythagoreans: these are a criterion for what counts and does not count as a substance (provided in Metaphysics Z.13-16) and the definition of unity in terms of indivisibility (formulated in I.1). On this basis, Aristotle claims not only that the one is not itself a substance, but also that it is not the substance of anything.

This dissertation has the following three main results. First, it demonstrates that Aristotle's dialectic cannot be regarded as monolithic but needs to be explored on a case-by-case basis. Second, it re-evaluates Aristotle's engagement with his predecessors, in particular the Eleatics. Third, it presents an argument which strongly tells in favour of a unitarian reading of Aristotle's Metaphysics.