Ontological arguments contend that we can know that God exists in the same way that we know that 2 + 2 = 4: through reason alone. In other words, they claim to show that God exists without ever consulting experience or observation. Anselm provided the first ontological argument for the existence of God in the late 11th century in his Proslogion, arguing that God must exist if we can conceive of him. In the mid-17th century Rene Descartes provided his own version of the ontological argument in Meditation 5. The classic version of this argument runs as follows:
1. God is the most perfect ('the greatest') being conceivable.
2. It is more perfect ('greater') to exist than not to exist.
3. Therefore, God must exist.
In the late-18th-century Critique of Pure Reason, Immanuel Kant provided the standard rebuttal to the classic ontological argument: the mere concept of what God is does not entail his existence. While we may conceive of God as having the property of being all-powerful (say), existing is not a property of a thing at all. (More specifically, existence is not a perfection.) So the second premise is false. God's existence concerns whether our concept of God corresponds to anything real, and pure reason cannot tell us that (unless the concept of God is self-contradictory, in which case God cannot exist). We can show that the classic ontological argument fails by keeping the erroneous second premise and replacing the first one with: "Utopia is the most perfect ('the greatest') society conceivable." The parallel conclusion that Utopia (or "the greatest car," or whatever) must exist is clearly false. Only observation could determine that such things exist.
In this paper Ryan Stringer discusses arguments from perfection, both for and against the existence of God. He begins with a simple argument from perfection for the existence of God and argues that it is unsuccessful. Then he defends two kinds of arguments from perfection against the existence of God. The first ones are inductive and thus present atheism as a tentative conclusion, while the second one is deductive and thus purports to conclusively demonstrate atheism based on the logical inconsistency between God's existence and the imperfect world in which we live.
"Descartes's fifth Meditation argument for God's existence relies on an untenable notion that existence is a perfection and that it can be predicated of God. I shall first explain what Descartes's argument for God's existence is, and then present his argument in propositional form. I will then attempt to support the argument that existence is neither a perfection nor a predicate of God."
Oppy draws attention to an objection to Godelian ontological arguments which has hitherto gone unnoticed.
In addition to evidential and logical arguments for atheism, there is a lesser-known third kind of argument. Modal arguments for atheism conclude that atheism is necessarily true on the basis of a mere possibility claim. In this paper Ryan Stringer considers how modal arguments for atheism contribute to the philosophical defense of atheism, concluding that modal arguments for atheism either (a) positively support atheism or (b) at least undermine modal arguments for theism.
In this online debate between Richard Carrier and Tom Wanchick, Carrier opens with a discussion of method followed by 5 arguments for naturalism and 2 arguments against theism, while Wanchick opens with 9 arguments for theism. In the first rebuttals, each debater criticizes the arguments offered by the other in the opening statements. In the second rebuttals, each debater defends their opening arguments against the criticisms of the other in the first rebuttals. Both closing statements focus on the purported deficiencies of the other debater's overall case.
An overview of the major ontological arguments for the existence of a god, that is, arguments drawn from analytic, a priori premises rather than observations of the natural world. Includes histories, taxonomies, objections to, characteristics of the ontological argument, and a bibliography. From the Stanford Encyclopedia of Philosophy.
Oppy provides a "general ground" for rejecting modal theistic arguments, arguments for the existence of God which makes use of the premise that God is a being who exists in every possible world.
Oppy criticizes mereological ontological arguments for pantheism.
A refutation of Makin's defense of "Anselm's Ontological Argument."
Jordan Howard Sobel's Logic and Theism is long, abstruse, and technical, but valuable for those who have an interest in its topics. Those looking for arguments based on empirical phenomena said to be best explained by the God hypothesis should look elsewhere. Sobel's focus is, rather, issues of definition and logical structure. He addresses everything from the ontological argument to the fine-tuning argument, demolishing all of the main arguments for God's existence. Moreover, he argues that the kind of omnipotence and omniscience that theists ascribe to God is incoherent, and defends both evidential and logical arguments from evil against the existence of God. Finally, he turns to a discussion of practical reasons for belief in God, such as those invoked by Pascal's wager. No cutting-edge research on these topics should omit Sobel's work.