Everything that changes
Thomas Aquinas's first way: the argument from change.
Everything around us changes. Water gets warm, seeds become trees, and we ourselves grow and age. Thomas Aquinas calls the argument from change the first and most evident of his five ways to God (Summa theologiae I q.2 a.3).
The argument rests on the concepts of act and potency: what a thing actually is, and what it can become. 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 change.
- To change is to go from being able to be something to actually being it, and that takes something that is already actual.
- Nothing can be both what changes and what is changed at the same time and in the same respect. So whatever changes is changed by something else.
- A chain in which each link acts only through the one before cannot go on to infinity. Then nothing would act at all.
- So there is something that changes things without itself changing, and this is what everyone calls God.
Concepts you needAct and potency · Cause
Act and potency actus, potentiaPotency is the capacity to become or receive something. Act is its actually being so. What receives something is in potency to it, and what is received is its act.
Example
A seed is a tree in potency, a grown tree is a tree in act. Cold water is hot in potency, until someone heats it.
In Thomas
Thomas applies the concepts to existence: what receives its existence is related to it as potency to act (De ente et essentia, ch. 3). In God there is no potency, and so his essence and his being are one (Summa theologiae I q.3 a.4).
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 is the difference between act and potency?
Potency is what a thing can become. Act is what it actually is. Cold wood is hot in potency, and becomes hot in act when the fire heats it.
Why can nothing change itself in the same respect?
It would have to have and lack the same thing at once. What makes the wood hot must already be hot, and what is being heated is not hot yet.
How do you answer someone who says animals move themselves?
One part moves another, as when the muscles pull on the legs. Nothing changes itself in the same respect and in the same way.
Why can't the chain go on to infinity?
Because each link acts right now through the one before, as the stick through the hand. Without something first that acts of itself, nothing acts, however many links you add.
What do you say to the question of what set God in motion?
The argument only says that what changes needs something to change it. God is pure act, has no possibility left to realise and so cannot change.