 One example that does not readily lend itself to these approaches is the problem of the Christian Trinity. According to this doctrine, God the Father, Jesus, and the Holy Spirit are three persons but one God or one substance. Thus they provide an alleged example where b and c are distinct persons but the same substance. We will not enter the debate, which is been conducted for over a millennium, as to whether this doctrine is coherent, but refer the reader to a modern discussion in Cartwright 1987. It is certainly accurate to say that all of Geach’s examples are contested. For details on how the problematic character of such examples can be systematically dispelled, see, for instance, Wiggins (2001, chap. 2).