The Cosmological Argument

Reasoning from the world to God

The cosmological argument asks why anything exists or changes at all. It starts with features of the world that we can observe, then reasons towards an ultimate explanation. Christians who accept it identify that explanation as God. It is a philosophical argument: it uses observation and reasoning rather than requiring someone first to accept the authority of the Bible.

A tree, for example, depends on a seed, water and other conditions for its growth. Explaining those conditions leads to further questions about their causes. The cosmological argument asks whether a chain of dependent things can fully explain itself, or whether it needs something that does not depend on anything else.

Thomas Aquinas set out five ways of arguing for God’s existence in his Summa Theologiae. His First Three Ways concern motion, causation, and possibility and necessity. They offer related but distinct routes to God as the ultimate foundation of the world.

The First Way: motion and change

Aquinas begins with the observation that things move or change. Here, motion means more than travelling from one place to another: it includes becoming different. Wood heated by a fire changes from potentially hot to actually hot. The fire brings about that change.

Aquinas argues that something cannot bring itself from potential to actual in the same respect: something already actual must bring about the change. If that mover also depends on another mover, a further explanation is needed.

His example is a hand moving a staff. The staff moves something else only because the hand moves it. Aquinas argues that a chain of dependent movers cannot explain motion without an ultimate source of movement. He therefore concludes that there must be an unmoved mover, which he identifies as God.

This is not simply a claim about the first movement long ago. It concerns the foundation on which other movers depend.

The Second Way: efficient causes

An efficient cause is something that produces an effect. Aquinas observes an order of causes in the world and argues that nothing can be the efficient cause of itself: it would have to exist before itself in order to produce itself.

He then rejects an infinite regress of dependent causes. In an ordered chain, intermediate causes produce effects because they receive their causal power from other causes. Aquinas argues that without a first cause there would be no intermediate causes or resulting effects. Yet effects clearly exist.

He concludes that there must be a first, uncaused cause, which he calls God. The difference from the First Way is the starting point: the First Way focuses on change; the Second focuses on causes producing effects.

The Third Way: contingency and necessity

Many things come into existence and later cease to exist. A tree grows and eventually dies. Such things are contingent: they can exist or fail to exist, and their existence depends on other things.

Aquinas attempts to argue that if everything were contingent, there would actually have been a time when nothing existed. His reasoning needs this premise about an actual empty time, not merely the possibility of one. He then argues that if absolutely nothing had existed, nothing could have begun to exist, because there would have been nothing already existing to bring it about. Since things exist now, he concludes that not everything can be contingent: something must exist necessarily.

The disputed step is the inference to an actual time when everything was absent. Each thing’s ability to fail to exist does not show that all things were absent together. Their periods of existence could overlap. Nor does the possibility of an empty past show that it actually occurred. Present existence therefore does not, by itself, establish a necessary being: Aquinas’ conclusion depends on accepting this contested premise.

He then asks whether a necessary being receives its necessity from another. Rejecting an endless chain of borrowed necessity, he concludes that there must be a being whose existence is necessary independently. Christians identify this necessary being as God.

The central contrast is between things whose existence depends on something else and an ultimate being that does not depend on anything else.

What might the argument reveal about God?

For Christians who accept these arguments, God is not merely another object within the universe. God is its ultimate source: the unmoved mover, uncaused cause and independently necessary being. This supports belief in God as eternal, self-existent and transcendent. Causing the universe also suggests immense divine power.

Some Christians regard the argument as a convincing demonstration of God’s existence. Others regard it as support for faith rather than a complete proof of everything Christianity teaches about God.

This distinction matters. An ultimate cause does not, by itself, establish that God is loving, that God is the Trinity, or that Jesus is God incarnate. Christians connect the argument’s conclusion with their wider faith, including revelation. Aquinas himself distinguishes demonstrating that God exists from knowing God’s nature perfectly: finite effects cannot give complete knowledge of an infinite God.

The objection: who caused God?

An atheist or Humanist might object that if everything needs a cause, God must need one too. Exempting God could look like an unjustified exception.

A Christian response is that Aquinas does not claim that absolutely everything needs a cause. His arguments concern things that change, are caused or have dependent existence. God is proposed as an uncaused, independently necessary being, not another dependent link in the chain.

This answers the charge of a straightforward contradiction. However, a critic can still ask why God, rather than the universe itself, should be accepted as the independent reality. Defining God as necessary is not enough on its own: the reasoning must show why such a being is needed.

Does the universe need an explanation beyond itself?

David Hume and Bertrand Russell challenge the move from individual things to the universe as a whole. Even if things within the universe have causes, it does not automatically follow that the whole universe has a separate cause. Russell’s position allows the universe simply to exist without a further explanation.

Critics may also question whether an infinite chain of causes is impossible. Aquinas’ rejection of infinite regress is an important part of his reasoning, not something every thinker accepts.

Christians respond that an endless collection of dependent things still leaves their existence unexplained. Adding more dependent causes does not supply an independent foundation. Even an everlasting universe could prompt the question of why it exists at all.

The disagreement therefore concerns what counts as a satisfactory explanation. Christians seek an ultimate foundation; critics question whether such a foundation is required or whether it must be God.

Science and a justified conclusion

Some atheists and Humanists argue that natural explanations remove the need to introduce God. The Big Bang theory provides a scientific account of the universe’s early development, whereas invoking God may appear to add an unsupported explanation.

Some Christians see the Big Bang as compatible with creation and as supporting belief that the universe had a beginning. Others emphasise that a scientific account of how the universe developed does not settle why the universe or its laws exist. God could be understood as the ultimate cause of a universe that develops through natural processes.

Neither interpretation makes the Big Bang decisive proof. A reasonable judgement might be that the cosmological argument provides a serious reason for belief because it addresses the dependence of the world, but does not conclusively establish the Christian God. Alternatively, someone might reject its demand for an ultimate cause. A justified conclusion should explain which disputed step is convincing or unconvincing, rather than merely announce agreement or disagreement.