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Aquinas' Five Ways
c. 1266–1268 (Summa Theologiae, Part I)· Idea or argument· Reality, mind & God
Well documentedSolid records, texts or physical evidence.
Five short arguments in the Summa Theologiae that reason from observable features of the world to the existence of God.
See it on the Philosophy map →The idea
The idea: start from what anyone can see and reason back to its cause. (1) Things change, so there must be a first unmoved mover. (2) Causes depend on causes, so there must be a first cause. (3) Things come and go, so something must exist necessarily. (4) Things are more or less good, implying a maximum. (5) Unthinking things act toward goals, implying a director. Each ends 'and this everyone calls God'. The main objection: why can't a chain of causes run back forever?
Why it matters
They remain the classic cosmological and design arguments, taught in every philosophy-of-religion course, and contrast with Anselm's attempt to prove God from the idea alone.
Where it comes from
They fill one article, Summa Theologiae Part I, question 2, article 3, written about 1266–1268; most draw directly on Aristotle and Avicenna.
Still debated
Aquinas did not deny an infinite past; scholars debate whether his chains mean causes stacked at one moment, like a hand moving a stick moving a stone.
Connections (3)
▸Aristotle's four causes underpin Aquinas' Five WaysMostly agreed
The connection
The Five Ways reason with Aristotle's types of explanation. The first two use efficient causes, what brings change about, to argue back to a first mover and a first cause. The fifth uses final causes, the goals things naturally tend toward, to argue that something intelligent directs nature.
When and how
Summa Theologiae, written about 1266–1268 in Rome; Aquinas drew on Aristotle's Physics and Metaphysics, where the unmoved mover first appears some 1,600 years earlier.
How sure are we?
The texts line up closely: the First Way restates the argument for an unmoved mover in Aristotle's Physics, Book VIII, on which Aquinas wrote a commentary.
Debates and open questions
Whether the Five Ways survive the loss of Aristotle's physics, such as his claim that every motion needs a mover, is still argued by philosophers of religion.
▸Thomas Aquinas formulated Aquinas' Five WaysWell documented
The connection
Aquinas rejected Anselm's proof from the mere idea of God, arguing that we must reason from effects we can observe. In one compact article of the Summa Theologiae he set out five arguments, from motion, causation, contingency, degrees of perfection and purposeful order, each ending at something 'everyone calls God'.
When and how
Rome, at the Dominican house of Santa Sabina, about 1266–1268, as he began the Summa Theologiae for beginning theology students.
How we know
The text survives as Summa Theologiae Part I, question 2, article 3; the first article of the same question explicitly rejects Anselm's approach.
Debates and open questions
Scholars debate why he wrote them so briefly; he gave fuller versions elsewhere, as in the Summa contra Gentiles, and may have meant a summary for students.
▸Does God exist? was answered from experience by Aquinas' Five WaysWell documented
The connection
The Five Ways are the classic answer to whether God exists that starts from experience rather than from definitions. They set the template for later a posteriori (experience-based) cosmological and design arguments, and framed the debate in which Hume and Kant argued that reasoning from the world cannot reach a perfect God.
When and how
Summa Theologiae, about 1266–1268, written in contrast to Anselm's ontological argument of 1077–1078 and drawing on Aristotle, Avicenna and Maimonides.
How we know
Aquinas explicitly poses the question 'whether God exists' and answers that 'it can be proved in five ways'; modern textbooks treat them as standard.
Debates and open questions
Whether they prove only a first cause or designer, rather than the God of any religion, has been disputed since Hume and is still argued.