What Are the Five Ways of St Thomas Aquinas?
The Short Answer
The Five Ways are five short arguments for the existence of God that St Thomas Aquinas set out in the Summa Theologiae (I, q. 2, a. 3). Each starts from something we can observe, such as change, causes, things that need not exist, degrees of goodness and order in nature, and reasons back to a source that explains it. Aquinas opens simply: “The existence of God can be proved in five ways.” They are not arguments about the distant past, but about what holds the world in being right now.
Critics often say the Five Ways are outdated, or that they rest on a simple mistake. Most such criticisms attack a version of the arguments Aquinas never made.
What the Five Ways Do and Do Not Claim
They do not claim:
- That everything needs a cause. They claim that things which change, depend or need not exist need a cause. God does none of these.
- That the universe must have had a beginning. Aquinas held that reason alone cannot prove that the world began (Summa Theologiae I, q. 46, a. 2). The Five Ways work even if it had always existed.
- That they prove the Trinity or the Gospel. They reach a first cause, a necessary being, a supreme good and a guiding intelligence. Who that God is, and what He has done for us, is known by revelation.
- That they are scientific theories. They are philosophical arguments from general features of the world, and no new discovery in physics makes those features go away.
They do claim that the existence of God can be known from His effects, by reason, as St Paul says: God’s “invisible nature, namely, his eternal power and deity, has been clearly perceived in the things that have been made” (Romans 1:20).
The Five Ways in Plain Language
1. Change
“The first and more manifest way is the argument from motion.” By motion Aquinas means any change: water heating, a seed growing, a mind learning. Nothing changes itself from what it could be into what it is; something already actual must bring that about. A hand moves a stick, which moves a stone. The stone moves only while the stick moves it, and the stick only while the hand moves it. In a chain like that, the middle links only pass the motion along. Without a first mover that is not itself moved, nothing in the chain would move at all. “Therefore it is necessary to arrive at a first mover, put in motion by no other; and this everyone understands to be God.”
2. Causes
The same reasoning applies to causes. You depend for your existence right now on air, on the earth, on the laws of physics, and they depend on something further. A chain of causes that are all dependent cannot explain itself, however long it is. There must be a first cause that depends on nothing.
3. Things that need not exist
Everything we see could have not existed. Its existence is borrowed. If everything had borrowed existence, there would be nothing to borrow it from, and nothing would exist. But things do exist. So there must be something that exists necessarily, of itself, and gives existence to everything else.
4. Degrees of goodness
We say one person is more honest than another, one act more just, one thing truer or nobler. Aquinas argues that “more” and “less” are measured against a real standard, and that the highest in goodness and being is the cause of goodness and being in everything else. This way is the least familiar to modern readers, but it rests on a real point: if goodness is not real, our rankings of good and evil are only preferences.
5. Order in nature
Things without minds act regularly toward ends. An acorn becomes an oak, not a cat; atoms bond in stable ways. A thing without knowledge can only tend toward a goal if it is directed by something with knowledge, as an arrow is aimed by an archer. “Therefore some intelligent being exists by whom all natural things are directed to their end; and this being we call God.”
Common Objections
”If everything needs a cause, what caused God?”
Aquinas never said everything needs a cause. He said that what changes, depends or need not exist needs a cause. A being that is unchanging, dependent on nothing and existing of itself is exactly what the arguments conclude to. (See If God Created Everything, Who Created God?)
”Why can’t the chain go back forever?”
For a chain stretching back in time, Aquinas agreed that reason cannot rule it out. But the Five Ways are about a chain operating now, like the hand, stick and stone. In that kind of chain, adding more links that only pass things along never explains where the motion comes from, any more than a longer train explains how carriages move without an engine.
”Even if there is a first cause, why call it God?”
Aquinas does not stop at the Five Ways. In the questions that follow he argues that the first cause must be simple, perfect, good, infinite, unchanging, eternal and one. The phrase “this everyone understands to be God” sums up a conclusion he then spends many pages defending.
”Evolution has explained away the fifth way.”
Evolution describes how living things develop over time. It does not explain why nature has stable laws at all, or why things tend reliably toward ends, which is what the fifth way is about. Evolution itself depends on that order. (See Does the Church Accept Evolution?)
In Summary
The Five Ways argue from change, causes, contingency, degrees of goodness and order in nature to a God who holds all things in being. They are about the present, not just the distant past, and they do not claim that everything needs a cause. They do not prove the whole faith, but they show that belief in God is a conclusion of reason, not a leap in the dark. The Five Ways bring you to God; the Gospels introduce you to Him.
See also Does God Exist? and, for a deeper treatment, Reading Aquinas’s Five Ways Rightly
This article follows the method St Robert Bellarmine used in his Controversies*: state the question fairly, clear away misunderstandings, make the case, then answer each objection. See How We Answer Objections.*
Sources
- Scripture: Revised Standard Version, Second Catholic Edition (Ignatius Press). Romans 1:20.
- St Thomas Aquinas, Summa Theologiae I, q. 2, a. 3; q. 3–11; q. 46, a. 2. Tr. Fathers of the English Dominican Province. ccel.org
- Catechism of the Catholic Church, nn. 31–36.