Argument from contingency

Argument from contingency

Also called	Third way; tertia via
Principal locus	Summa theologiae I, q. 2, a. 3
Kind	A posteriori argument from possible being
Doctrinal standing	A Thomistic demonstration serving the dogma that God can be known from creatures; the technical form is school theology

The argument from contingency is the a posteriori demonstration, classically stated as Saint Thomas Aquinas's third way, that from beings which can be and not-be one must arrive at a being necessary through itself, which all call God.

Authority level: Dogma

Possible being and necessary being

Contingent being, in the sense Thomas uses here, is that which can be and can not-be. Generated and corruptible things make this plain: they begin, they cease, and while they last they do not contain in themselves the reason why they exist rather than not. From the fact that such things now exist, it does not follow that they exist of themselves. If everything were of that kind, then at some time there would have been nothing, and from nothing nothing comes. Therefore there must be something whose existence is necessary.

Necessity here is not a modern modal slogan and not a laboratory measurement. It names a being that cannot not-be. Among necessary things, some may have their necessity from another. The argument does not stop at a necessary creature. It asks for that which is necessary through itself, the uncaused source of necessity in others. That terminus is God. The third way is therefore a causal argument about existence, not a claim that every empirical event has already been catalogued.

The third way in the Summa

In Summa theologiae I, question 2, article 3, Thomas places the third way among five ways of showing that God exists. The first argues from motion, the second from efficient causality, the third from possibility and necessity, the fourth from degrees of perfection, the fifth from the governance of things. The third is not a restatement of the second in other words. Efficient causality asks who produced this effect. Contingency asks why there is a being that need not exist at all. The two questions meet in God, but they start from different features of creatures.

The Summa's formulation is brief and assumes the metaphysics developed elsewhere: act and potency, received existence, and the impossibility of an infinite regress of caused necessity. Readers who extract the third way as a stand-alone puzzle, without that grammar, often accuse it of smuggling what it should prove. Thomas is not offering a modern cosmological argument from the Big Bang, nor Leibniz's principle of sufficient reason in seventeenth-century dress. He is arguing from the actual existence of generable things to a first necessary being.

What Catholic doctrine requires

Dei Filius teaches that God, the beginning and end of all things, can be known with certainty from created things by the natural light of human reason. The Catechism repeats that teaching and notes how sin, finitude, and disordered culture make the knowledge difficult in practice. Lateran IV and the same Vatican constitution confess one living God, Creator of all, really distinct from the world. Those are the defined measures. No council defined the wording of the tertia via.

The argument from contingency therefore serves dogma without being dogma. It is one classical way of making good on what the Church says reason can do. A Catholic may find another valid a posteriori path more persuasive, or may know God as Creator principally by faith while still holding that reason is competent. What may not be held is that the world's existence is underivable from God, or that the Creator is a being among beings who happens to be larger.

School form and later names

Later Scholastics and modern philosophers have recast contingency arguments in other idioms: possible worlds, sufficient reason, or the supposed necessity of a first moment of time. Some of those recastings are useful. None should be back-read into Thomas as if he had written them. The third way does not prove that the world began; Thomas held that a beginning of time is known by faith, not demonstrated. Contingency concerns dependence in being, not a dated first frame of the cosmos.

Thomists also distinguish this demonstration from the fifth way, which argues from order to an intelligence that directs, and from purely physical cosmology, which studies secondary causes. Aeterni Patris and Fides et Ratio commend Aquinas as a master of this labor. They do not forbid other faithful philosophies from arguing the same Creator by other valid means. The third way remains a privileged Thomistic instrument: exact, limited, and ordered to worship rather than to winning a puzzle.