The argument from design is an a posteriori argument. That is, it seeks to prove its conclusion by investigating the world. In addition to a posteriori arguments there is also another kind of argument, an a priori argument. An a priori argument seeks to prove its conclusion just by analyzing concepts using the faculty of reason. Because Hume is an empiricist he does not believe that we can ever prove any matters of fact using a priori arguments. However, he nonetheless devotes a chapter of his book to attacking the most famous a priori argument for the existence of God: the ontological argument.

The ontological argument comes in many forms. The first person to propose a version of the argument was the medieval philosopher St. Anselm. Other famous versions have been put forward by René Descartes, Baruch Spinoza, and G.W. Leibniz. In Dialogues Concerning Natural Religion it is Demea who suggests that a version of the ontological argument might serve as a plausible alternative to the flailing argument from design.

The version of the ontological argument that Demea puts forward goes as follows:

(1) Every effect has some cause.
(2) Therefore, there must either be an infinite chain of causes or else there must be some ultimate cause that is its own reason for being (i.e. a necessarily existing thing).
(3) There cannot be an infinite chain of causes because then there would be no reason why that particular chain exists and not some other, or none at all.
(4) Therefore, there must be a necessarily existing thing, i.e. God.

Both Cleanthes and Philo have a field day ripping into this argument. Cleanthes argues, first of all, that matters of fact cannot be proved a priori, and shows why this is the case. He also objects that the argument only proves that there is some necessarily existing thing and that this necessarily existing thing could just as easily be the material world as it could be God (neither would be more inexplicable and mysterious than the other). In addition, he mentions, there is actually no good reason why there cannot be an infinite chain of causes. Philo then steps in with an added objection: for all we know, he says, there is some necessity to the material world that we do not understand. There might be some laws that explain everything without recourse to a necessarily existing being.

By arguing against the ontological argument (and, in the process, against all a priori theological arguments), Hume successfully covers all of his bases. Without any a posteriori arguments, and without any a priori arguments, there can be no rational basis for religious belief. Neither reason nor experience can justify a belief in God's nature.

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