The Christian philosopher and logician Peter Geach (1916–2013) developed a way to make sense of the idea of the Holy Trinity: the thesis that the Father, the Son, and the Holy Spirit are all one and the same individual. Geach’s solution to the conundrum of how three apparently different individuals with seemingly different properties can in fact be one is known as ‘relative identity’. This is because Geach’s idea is to say that two individuals, a and b, might be the same kind of thing F without being the same kind of thing G. (In other words, some a and some b might be the same relative to some F, but not relative to some G. Applied to the Trinity, and in more concrete terms, we could say that the Father and the Son are the same god-thing but not the same person thing (even though each is also a person).
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