Assessing the case for theism › Can a concept establish existence?

Ontological arguments start from a conception of God rather than from observations of the world. Mackie's chapter 3 treats several versions, so its objections should not be reduced to a single slogan about existence. A definition can tell us what would qualify as a certain kind of being; the further claim that such a being exists needs defense.

Plantinga's modal version sharpens the issue. In the framework Mackie considers, a maximally great being would possess the relevant excellence in every possible world. These worlds represent ways things could have been, not planets elsewhere in space. Given that framework, the possibility of such a being carries a strong consequence: its existence cannot merely be contingent.

Mackie grants the inference for the purpose of his main objection. He asks why someone not already persuaded of theism should accept the premise that maximal greatness is possibly instantiated. Saying that we have not noticed a contradiction does not settle every question about metaphysical possibility. His criticism concerns the premise's independent justification, rather than dismissing modal reasoning as meaningless. Chapter 3(c), especially pp. 57–61.

This is a useful reading habit for any elaborate argument. Formal precision can show exactly what follows from an assumption. It cannot by itself supply a reason to accept that assumption. When an impressive conclusion depends on one controversial premise, spend your attention there.