Argument from contingency
The argument from contingency is a cosmological argument proposed by Thomas Aquinas in his book Summa Theologica. Because it is his third argument, it is also known as Aquinas's third way. It argues that some objects have the property that they must exist, because if everything is contingent (it might exist or it might not) and transient, there would have been a state in which nothing existed at all, which is supposedly absurd.
- "You can’t just have an infinite number of things in Box A [contingent things], each requiring other things in Box A in order to come into existence. If that were the case, nothing would ever exist. Things do exist, so we know that can’t be the right answer. [... this] shows that Box B [necessary things] can’t be empty."
- Natural objects tend to have been generated and have a tendency/possibility to be corrupted. 
- From (1) These objects may exist or may not exist i.e. they are contingent.
- If an object can be non-existent and tends to be corrupted, each object sometimes does not exist.
- From (3), contingent objects cannot always exist i.e. they are transient.
- If everything is contingent and tends to be corrupted or have been generated, then at some point nothing existed at all.
- Contingent objects require something that exists to bring it into existence.
- If nothing existed in the past, nothing contingent would exist now.
- Contingent things exist.
- Therefore not everything is contingent.
- Some objects are not contingent. These are necessary objects.
Having established that there must be necessary objects, the argument moves to consider causes of necessary objects.
- Necessary objects are cause by another necessary object, or not.
- There is no infinite regress of necessary objects causing other necessary objects.
- Therefore the chain of causes terminates in a necessary object that itself its own necessity, i.e. God.
- "The “possibility” in question is not some abstract logical possibility but rather something “inherent,” a tendency “to be corrupted” rooted “in the nature of those things… whose matter is subject to contrariety of forms” (QDP 5.3). In other words, given that the matter out of which the things of our experience is composed is always inherently capable of taking on forms different from the ones it happens currently to instantiate, these things have a kind of inherent metaphysical instability that guarantees that they will at some point fail to exist."
In a sense, the argument is based on the tendency of objects not to exist.
Contingency and transiency does not imply the past non-existence of everything
Aquinas points out that individual objects come into existence and decay out of existence, implicitly saying they tend to not remain in existence. In other words, a individual objects "tend not to exist". However, the tendency of objects to non-existence does not generalise to all objects tending to non-exist at the same time. His point would be valid if existence and non-existence of objects was random and the universe had finite material. Looking far back in time, everything would non-existent by chance and that "everything non-existent" state could not kick start history. But this is not true in general because existence and non-existence is not random.
Objects are mental constructs, the material of an object is more fundamental. While a house may have been created, it was built out of pre-existing trees. If the house burns down, it is destroyed but it creates ash. In each case the materials, or to be specific, the atoms and energy that constituted each object continues exist in another form. Objects are better understood to be in flux or transition to other objects. Aquinas argues that objects are "destroyed" but this is hardly relevant when the materials persist. The universe tends to have conservation laws, such as the conservation of mass and of energy. This seems a much more universal principle than Aquinas's claim of contingency and transience.
Based on our experience, the materials in the universe continued to exist, in various forms. We can therefore suppose the materials have always existed, perhaps in different forms or in unknown forms.
Assumption that an infinite regress cannot happen
- Main Article: Infinite regress does not occur
The argument assumes infinite regress cannot occur but this is difficult to establish if it is true.
Natural processes are not ruled out
Natural processes are not ruled out. The universe or some physical process might have the property of necessarily existing.
No specific God is supported by the argument
- Main Article: The first cause implies God exists
No specific God is supported by the argument and the attributes of God cannot be inferred. The conclusion is hardly relevant to religion. The argument does not rule out polytheism or pantheism.
Proof by logic
- Main Article: Proof by logic
Pure logic proofs cannot say anything about matters of fact.
Objects may spontaneously come into existence
- Main Article: Not all events necessarily have causes
The argument asserts that "contingent objects require something that exists to bring it into existence." However, this is arguably a false statement and a hasty generalization. It is possible that some events, particular on the quantum scale, do not have causes (or at least we do not fully understand the cause at this time). This is also known as the Glendower problem.
- Why is there something rather than nothing?
- Argument from continued existence
- Existence of finite beings prove that infinite beings exists