Aquinas's Five Ways — presented in Summa Theologiae I.2.3 — are five distinct arguments for God's existence drawn from features of the observable world. The First Way argues from motion: everything moved requires a mover, and since this chain cannot regress infinitely, there must be an unmoved first mover. The Second Way argues from efficient causation: no cause can cause itself, and an infinite regress of causes is impossible, so there must be a first efficient cause. The Third Way argues from contingency: things that exist contingently (they could not exist) require a necessary being that cannot not exist — the ultimate ground of all contingent existence. The Fourth Way argues from degrees of perfection: gradations of goodness, truth, and nobility in things require a maximum that is the cause of all lesser instances. The Fifth Way argues from design: unintelligent things consistently act toward ends, which implies an intelligent director governing natural processes. Each of these converges on what Aquinas identifies with God. The arguments depend on Aristotelian metaphysics; post-Kantian philosophy largely rejected them while analytic philosophy has partially rehabilitated their key premises.
The First Way from motion: "It is certain, and evident to our senses, that in the world some things are in motion... Whatever is in motion must be put in motion by another... therefore it is necessary to arrive at a first mover, put in motion by no other; and this everyone understands to be God."
The Second Way from efficient causation: "There is no case known in which a thing is found to be the efficient cause of itself... therefore it is necessary to admit a first efficient cause, to which everyone gives the name of God."
The Third Way from contingency: "We find in nature things that are possible to be and not to be... if everything is possible not to be, then at one time there could have been nothing in existence... Therefore we cannot but postulate the existence of some being having of itself its own necessity."
The Fourth Way from degrees of perfection: "Among beings there are some more and some less good, true, noble... there is something which is truest, something best, something noblest... Therefore there must also be something which is to all beings the cause of their being, goodness, and every other perfection; and this we call God."
The Fifth Way from design: "We see that things which lack intelligence, such as natural bodies, act for an end... Therefore some intelligent being exists by whom all natural things are directed to their end; and this being we call God."
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