Alvin Plantinga (1932– ) is an American philosopher, and professor at the University of Notre Dame. He is known for his contributions to epistemology and metaphysics, and, as a Christian, for his philosophy of religion and defense of Christian beliefs.

Christian philosophy

Plantinga argues that one can have knowledge of God without justification, in the same way that one can have knowledge of the existence of other minds. He argues that one may doubt both from scepticism, but ultimately one must accept both in order to be consistent.

Additionally, Plantinga also argues that there is no “problem of evil”, and that there is no logical contradiction between the existence of an omnipoetent, benevolent god and the evil that occurs in the world. Plantinga argues that God created human beings with free will, and that free will is necessary for good to exist. Thus, in order for there to be good, God must allow some evil to exist in the world, otherwise there would be no free creatures capable of moral good.

Modal Ontological Argument

Plantinga has offered one of many formalizations of Anselm’s ontological argument in modal logic, the logic of neccessity and possibility. His argument may be summarized as follows:

1. Premise: In some possible world, Δ, a being has maximal excellence if, and only if, that being is omnipotent, omniscient and omnibenevolent at Δ
2. Premise: A being has maximal greatness if it has maximal excellence at every possible world.
3. Premise: Maximal greatness is possible.
4. So, it is possibly neccessary that an omnipotent, omniscient and omnibenevolent being exists.
5. So, it is necessary that an omnipotent, omniscient and omnibenevolent being exists.
6. Therefore, an omnipotent, omniscient and omnibenevolent exists.

Plantinga’s argument assumes a modal logic of S5, in which (4) is possible—that is, things that are possibly neccessary are necessary. The main objection to his argument, however, is with (3), that maximal greatness is possible as Plantinga has defined it.

Selected Works

• God and Other Minds (1967)
• The Nature of Necessity (1974)
• Warranted Christian Belief (2000)