What could fail to exist
Thomas Aquinas's third way: the argument from the possible and the necessary.
Flowers wither, mountains erode and stars burn out. Everything we meet can exist, but it can also not exist. Thomas Aquinas's third way to God (Summa theologiae I q.2 a.3) asks what that tells us about why anything exists at all.
Read the argument in short first. Then walk through it step by step, and at each step pick the most common objection to see how to answer it.
The argument in short
The argument in five sentences, to learn by heart.
- Things come into being and pass away. They can exist, but they can also not exist.
- If everything were like that, at some time there would have been nothing. Then nothing would exist now either, for what does not exist can only begin to exist through something that exists.
- So there is something that cannot fail to exist.
- It has that either from something else or from itself, and a chain of things that have it from something else cannot go on to infinity.
- So there is something necessary in itself that gives everything else its existence, and this everyone calls God.
Concepts you needEssence and existence · Cause
Essence and existence essentia, esseEssence is what a thing is. Existence, or being, is that it is. In everything created the two are distinct: existence is not part of what the thing is.
Example
You know what a rose is without knowing whether there is a rose on the table. To know that, you have to look.
In Thomas
Thomas discusses the distinction in De ente et essentia, ch. 3 (ch. 4 in some editions). In Summa theologiae I q.3 a.4 he shows that only in God are essence and being the same.
Cause causa fiendi, causa essendiA cause is what something depends on. Thomas distinguishes the cause of a thing coming to be from the cause of its being. He also distinguishes chains whose links merely follow one another from chains where each link acts right now through the one before.
Example
A builder is the cause of a house coming to be, but the house still stands when he goes home. The sun is the cause of the light in the air, and the light vanishes the moment the sun stops shining.
In Thomas
The builder and the sun are Thomas's own examples, in Summa theologiae I q.104 a.1. He writes about the two kinds of chain in I q.46 a.2, reply 7: a stone moved by a stick, moved by a hand, compared with generations where one man is begotten by another.
Step by step
or tap a box in the map to see the texts behind it.
Test yourself
Think of your answer first. Nothing is saved.
What does Thomas mean by what could fail to exist?
Things that come into being and pass away. They can exist, but they can also not exist. Today they are often called contingent.
Why can't everything be the kind of thing that can fail to exist?
Then at some time there would have been nothing. From nothing nothing can begin to exist, and then nothing would exist now either.
How do you answer someone who says the universe can come from nothing?
What physicists call nothing, such as a quantum vacuum, has energy and obeys laws. It is something. From truly nothing comes nothing.
Why can't the universe itself be what is necessary?
The universe is made of parts that come and go. Everything composite has a cause, and what is necessary in itself cannot be composite.
How do we know the different ways lead to the same God?
There can only be one God. If there were two, one would lack something the other has, but God lacks nothing.