John Watkins is a critical rationalist. I can think of no more appropriate way of celebrating his contribution to philosophy than by taking one of his conjectures and subjecting it to critical scrutiny: his account of the unity of theories.¹ But criticism is only part of critical rationalism, and it is only part (Section 1) of this paper. In Section 2 I rework this conjecture utilising a possible-worlds framework and a thoroughgoing realism concerning causation, law and theory. Most of the desiderata which Watkins lays down for a concept of unity appear to be satisfied within the new account. Moreover, it yields the serendipitous consequence that unified theories must also possess another virtue for which Watkins argues at length: depth. In Section 3 I offer an account of the aim of scientific inquiry which unifies the distinct elements of Watkins's account.