The Cosmological argument is ‘a posteriori’ – it is reliant upon and fits with our experience of the world around us, with our own experience of causal chains provoking questions over how we, and the universe as a whole, came to be. The aim of the Cosmological argument is to attempt to prove God’s existence by showing that an infinite regress of causal chains is logically impossible, and in turn, that there must have been a first cause. It highlights the problems of infinite regression and suggests God’s existence as a solution. There are several versions of the argument, the classic being that of St. Thomas Aquinas. Other significant figures include Leibniz and Kant. The Medieval philosopher, St. Thomas Aquinas, provided five arguments in his book, the Summa Theologica, the first three of which are cosmological: the argument from motion, the argument from causation and the argument from necessity and contingency. In his Second Way, the argument from causation, Aquinas argues that nothing causes itself, so if the universe was to exist – which it does -, there must be a first cause. It is an ‘a posteriori’ argument as it is from our own experience that we know causes are ordered in to causal chains. P1) There is an order of efficient causes (every event has a cause) P2) Nothing can be the cause of itself P3) Imagine this order of causes goes back infinitely – then there would be no First Cause P4) If (3) were true, then there would be no subsequent causes, but this is false. C) There must be a First Cause (the source of all causes) and this we call God Aquinas argues that nothing can cause itself because if something were the cause of itself it would have to be prior to itself, which is impossible. Therefore, if the universe was to exist at all there needs to be a first cause which began a chain of cause and effect. This argument is reductio ad absurdum. Aquinas attempts to argue that God must exist because an absurd result would follow from the denial of his existence; he uses premises three and four to demonstrate the impossibility of there being no first cause, by demonstrating that if there was no first cause then the present state would not exist – something which is obviously false. In this way, Aquinas’ argument from causation is reliant upon the idea that the universe cannot be infinite. In support of premises 3 and 4 of the argument from causation, Immanuel Kant also argues that an infinite chain of causes is something that, by definition, could never be completed; if the causes that lead up to the existence of us and the world really stretched off in to an infinite past, then there would have to be an infinity of causes occurring before the world could come to be. Kant argues that this is impossible, as if there were an infinite number of causes prior to the present state, then the present state could never come to be. Since the present state has come to be, there cannot be an infinity of causes and in turn there must be a first cause, which people call God. However, philosopher Gottfried Leibniz views the internal regress of the universe differently, with his principle of Sufficient Reason suggesting that since there does not appear to be anything within the universe itself to say why it exists, one can conclude that there is sufficient reason to believe in a great cause outside the universe. In this way, Leibniz argues that the uncaused causer must exist outside the series of causes, infinite though this series may be; he argues, in turn, that such causer must be in esse. Aquinas’ Third Way in the Summa Theologica is the argument from necessity and contingency. This is different from the arguments from motion and causation in that it is based upon the contingency of the universe and of everything in it. P1) Everything in the universe is contingent on something else. P2) Being contingent means that something need not exist P3) If everything need not exist, at some time they did not exist P4) If at one time nothing existed, nothing would exist today P5) Things exist today C) Therefore, there must be a non-contingent or necessary being to explain this (God) Here, Aquinas argues that since the universe is contingent, it cannot be the cause of its own existence and is not necessary. Therefore, a necessary being is required to bring the universe in to existence – where all things are contingent and in turn unnecessary, God is by contrast a necessary being as he is not contingent upon anything else. Another simpler version of the Cosmological Argument is the Kalam argument. This is an Islamic form of the argument which goes back to the Muslim philosopher Al-Kindi but which has also seen support from contemporary philosopher, William L Craig. P1) Everything that begins to exist has a cause of its existence P2) The universe began to exist C) Therefore, the universe has a cause of its existence In defence of premise two, ‘the universe began to exist’, Craig follows Kant’s reasoning that an actual infinite cannot exist; he states that a beginningless temporal series is an actual infinite, and that since an actual infinite cannot exist, a beginningless temporal series cannot exist. However, the Cosmological Argument also faces several objections. Perhaps the most obvious objection to Aquinas’ argument from causation, known as the schoolboy objection, is the criticism that premise one – ‘every event has a cause’ – and the conclusion – ‘there must be a First Cause which itself has no cause’ – seem to contradict each other. In response to this, it has been argued that there must be an exception to the rule and that this is proved by its reductio ad absurdum form; if there was no exception, no uncaused causer, then the universe would have no cause and it could never come to exist. There is, though, also a retort to this response to the objection. It could be argued that instead of God being the exception to the causal rule, the universe itself could be the exception, with only the things within it having to follow the rule. Furthermore, we could say that the existence of the universe does not need any further explanation: it simply is. A further objection comes from Hume, who, in line with his epistemology, suggests our cognitive inability to comprehend the nature of a metaphysical superentity powerful enough to bring the universe in to existence. In this way, he argues that we have no reason to infer what brought the universe in to existence. An unwarranted inference is required to claim that it is actually God. Hume also argues that the Cosmological Argument suffers from the fallacy of composition, an idea later supported by Russell. The fallacy of composition is the fallacy of assuming that since there is some property common to each part of a group, it follows that this property applies to the group as a whole. Therefore, although every individual part of the universe may have a cause, it does not necessarily follow that the universe itself must have a cause. To conclude, the Cosmological argument is an a posteriori argument whose aim is to attempt to prove the existence of God. Both the Kalam cosmological argument and those of St. Thomas Aquinas attempt to prove this existence through reductio ad absurdum means, demonstrating that without a first cause, the present state would be impossible. However, the strongest objection to the cosmological argument is as a result of its a posteriori basis; that what we see and experience, within the universe, must apply to the universe as a whole. Hume objects to the assumption that because that within the universe is subject to causation, the universe as a whole must also follow this rule. Although it can be argued this fallacy of composition does not always apply in every circumstance, there is no way of ensuring that it does not apply in terms of the whole universe, and in turn the soundness of the cosmological argument cannot be ensured.