The following forms of the three traditional arguments for God were copied from Philosophy of Religion: An Historical Introduction, by Linda Trinkous Zagzebski. As Zagzebski notes, these arguments were originally written to bolster the faith of those who already believed, rather than to convert non-believers. So it’s best to think of them as thought experiments, even if they have become debate topics in the modern world.
Paley’s Analogical Argument
(1) We observe in artifacts such as a watch order and regularity of parts.
(2) We know that a watch could not have these features without a designer, a conscious being who creates it intentionally.
(3) Nature itself exhibits order and regularity of parts.
(4) Therefore, nature must have a designer, a conscious being who created nature intentionally.
Hume’s Analogical Argument
(1) Nature is a great machine, composed of lesser machines, all of which exhibit order.
(2) Machines caused to exist by human minds exhibit order.
(3) Nature resembles machines caused to exist by human minds.
(4) If effects resemble each other, the causes do as well.
(5) So the cause of nature resembles human minds.
(6) Greater effects require greater causes.
(7) Nature is a much greater machine than the machines produced by human minds.
(8) So the cause of nature resembles but is much greater than human minds.
A Contemporary Probabilistic Argument
(1) The universe has a large number of life-facilitating coincidences between causally unrelated aspects of the physical universe. For example, the ratio of the density between an open universe that goes on expanding for ever and a closed universe that collapses upon itself is extremely narrow, and the density of the universe is in that range. In addition, if any of the fundamental physical constants (strong and weak nuclear forces, electromagnetic force, electron charge) had differed even minutely from what they in fact are, the universe would not have supported life. Intelligent life could only have evolved in an extremely narrow range of possible universes.
(2) The probability that this could have occurred by chance is infinitesimally low.
(3) Therefore, it is much more probable that our universe was intelligently designed than that it occurred by chance.
The Cosmological Argument
Clarke’s Argument from the Principle of Sufficient Reason
Necessary Being: A being that cannot not exist.
Contingent Being: A being that can not exist.
Principle of Sufficient Reason (PSR): Every contingent fact requires an explanation for its truth. (Presumably necessary facts are self-explanatory.)
(1) Every existent thing must be either contingent or necessary.
(2) Assume that everything in existence is contingent.
(3) The fact that the world of contingent things exists is contingent.
(4) The fact that the world of contingent things exists needs explanation (by PSR).
(5) The fact that there is a world of contingent things cannot be explained by something outside it since, by hypothesis, nothing else exists, but its existence could not be explained by anything inside it since the existence of a whole cannot be explained by the existence of a part.
(6) So if everything in existence is contingent, the existence of the world of contingent things has no explanation.
(7) Therefore, there must exist a necessary being whose existence explains the existence of the world of contingent things.
The Ontological Argument
Anselm’s Argument in Proslogion 2
G: That than which nothing greater can be conceived.
(1) We can conceive of G, which is to say, G exists at least in our understanding.
(2) Suppose that G does not exist in reality.
(3) We can conceive of G existing in reality.
(4) It is greater to exist in reality than to exist merely in the understanding.
(5) So we can conceive of G (i.e. that than which nothing greater can be conceived) being greater than it is.
But (5) is a contradiction.
(6) Therefore, the supposition (2) is false. G exists in reality.
A Contemporary Modal Ontological Argument
(1) It is possible that there is a being whose non-existence is impossible.
(2) Therefore, there is a being whose non-existence is impossible.
Even if one finds all of these arguments persuasive, none of them point toward any specific religion. Although these arguments were conceived of by various Jewish, Christian, and Muslim theologians, they can only be used to argue the existence of an undefined Being. Additionally, even theologians and devout philosophers debate each of these arguments. Emmanuel Kant objected to all of them, for example, despite his steadfast belief in God:
In the Critique of Pure Reason, Kant argued that the three classical arguments are the only three possible theoretical arguments for God’s existence and they all reduce to the Ontological Argument. Here’s why. The Teleological Argument is an analogy which only works if the universe is similar to a human artifact. But if the universe is similar to a human artifact, the argument proves only that the universe has a maker that is analogous to a human artificer. But humans don’t create anything. We can rearrange matter into a different form, but we don’t create the matter itself. Therefore, the Teleological Argument can prove at most that there is an architect of the universe, not a creator.
To prove that the architect of the universe is also a creator, we need a proof of the contingency of the universe, that it depends upon a necessary being that created it. But that is the Cosmological Argument. Now the Cosmological Argument is supposed to be based on experience, but it really is not. The only part of it that uses experience is the first premise, which says that contingent things exist. The argument then proceeds a priori to a necessarily existent being upon whom the contingent universe depends for its existence. But how do we know that such a being is the highest being, a perfect being? We need an argument that necessary existence is a perfection, and that is the Ontological Argument. So, Kant argues, the only real argument for the existence of God is the Ontological Argument, and as we saw above, Kant thinks the Ontological Argument fails. Kant’s own argument is one in which the existence of God is a postulate of practical reason, a demand of morality. (Zagzebski, 52-53.)
Earlier in the text…
The opposition to this [Ontological] argument generally focuses on premise (4). (See Anselm’s Argument in Proslogion 2.) This premise appears to maintain that existence is a great-making property. Kant disputed this on the grounds that existence is not a property at all, but even if it is, surely existence is not a great-making property of everything. For instance, we would not want to say that an existent terrorist is better than a non-existent one. (Zagzebski, 49-50.)