Thomas Aquinas and the Five Ways
St. Thomas Aquinas (c1225-1274) is arguably the most important Catholic theologian in history. In his major work Summa Theologica, widely considered as the highest achievement of medieval systematic theology, Aquinas presented his five proofs of God's existence known as the Quinque Viae (Latin for "Five Ways").  We will be presenting all the arguments in more detail a little later, at present we will give a brief rundown of all five arguments.
* Thomas' first way involves the evidence of motion. The fact, to Thomas, that every moving thing needs a mover shows that God, the Unmoved Mover, exists.
* The second way involves the notion of efficient cause. For the series of causes and effects, that we see in the world, to make sense it must have a beginning. God, the First Cause, therefore exists.
* The third way notes that every existing thing does not owe its existence to itself. However, if all things are contingent, there could not have been anything as at one time all these could be non-existent. To account for all existence, there must be a Necessary Being, God.
* The fourth way shows that there exist gradations in things, for example more noble and less noble, more true or less true. The existence of such gradations implies the existence of an Absolute Being as a datum for all these relative gradation.
* The fifth way argues that the behavior of things in the world implies a Grand Designer or architect, God. 
Thus Aquinas' five ways defined God as the Unmoved Mover, the First Cause, the Necessary Being, the Absolute Being and the Grand Designer.
It should be noted that Aquinas' arguments are based on some aspects of the sensible world. Aquinas' arguments are therefore a posteriori in nature. By contrast, Anselm's argument is based purely on an a priori definition of God.  Aquinas' Five Ways are based ultimately on sense experience. Sense experience can never be infallible. Thus by themselves these arguments cannot establish the existence of God with complete certainty. However, should his arguments be valid, the existence of God would be an established fact on par with many of the discoveries of modern science.
The Roman Catholic Church considers the first three ways of Aquinas (collectively called The "Cosmological Arguments" [a]) as conclusive evidence for establishing the existence of God.  The Catholic Church notwithstanding, we will now proceed to examine for ourselves the validity of Aquinas' arguments.
We will now see how none of the five ways prove the existence of God:
* The First Way: God, the Prime Mover
* The Second Way: God, the First Cause
* The Third Way: God, the Necessary Being
* The Fourth Way: God, the Absolute Being
* The Fifth Way: God, the Grand Designer
The First Way: God, the Prime Mover
In the first way, God is defined as the Prime Mover. We will let Aquinas speak for himself in explaining his first argument for the existence of God.
The first and most manifest way is the argument from motion. It is certain, and evident to our senses that in the world some things are in motion. Now whatever is moved is moved by another, for nothing can be moved except it is in potentiality to that towards which it is moved; whereas a thing moves inasmuch as it is in act. For motion is nothing else than the reduction of something from potentiality to actuality. But nothing can be reduced from potentiality to actuality, except by something in a state of actuality. Thus that which is actually hot, as fire, makes wood, which is potentially hot, to be actually hot, but only in different respects. For what is actually hot cannot simultaneously be potentially hot: but it is simultaneously potentially cold. It is therefore impossible that in the same respect and in the same way a thing should be both mover and moved; i.e. that it should move itself. Therefore, whoever is moved must be moved by another. If that by which it is moved be itself moved, then this also needs to be move by another, and that by another again. But this cannot go on to infinity, because then there would be no first mover, and consequently no mover, seeing that subsequent movers move only inasmuch that they are moved by the first mover; as the staff move only because it is moved by the hand. Therefore it is necessary to arrive at a first mover, moved by no other; and this everyone understands to be God. 
With the benefit of modern physics we can get rid of this argument for good. Newton's First Law states that a particle would tend to stay at rest or move in a constant velocity if no external force is applied to it. Hence it is as natural for a body to move (in a constant velocity) as it is for a body to be at rest. There is no need for a Prime Mover at all.
Aquinas' physics was, of course, based on Aristotle's, which states that the natural state of any body is to be at rest. Furthermore, the experiments done by physicists A.A. Michelson (1852-1931) and E.W. Morley (1838-1923) in 1881 (and again in 1887) showed that there was no standard and absolute frame of reference in the universe. Albert Einstein (1879-1955) used the conclusion of this experiment as one of his postulate in his Special Theory of Relativity. What this experiment and Einstein's theory showed was that there is no such thing as absolute motion. All velocities can only be given relative to something else, none of which is an absolute reference. This, of course, makes the idea of a Prime Mover absolute nonsense!
There is another, more traditional, objection to the first way. The fundamental principle in the argument is that everything which initiates change must have been initiated in some way itself. This principle, must therefore be applied to the Prime Mover as well. There is no logical reason why we should stop applying that principle at that point. This objection is conclusive. For the argument begins with an observation (that there is motion) and a fundamental principle (that every moving thing is moved by another already moving) which, at least in Thomas' philosophy, seems to be valid. Yet he postulated the existence of The Unmoved Mover that violates the argument's own fundamental premise. The irritating (to believers) question of a naturally skeptical child sums up the main problem with the first way: If God made the world, who made God? 
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The Second Way: God, the First Cause
In the second way, God is defined as the First Cause, as Aquinas elaborates:
The second way is from the nature of efficient cause.[b] In the world of sensible things we find there is an order of efficient causes. There is no case known (neither is it, indeed, possible) in which a thing is found to be the efficient cause in itself; for so it would be prior to itself, which is impossible. Now in efficient causes it is not possible to go on to infinity, because in all efficient causes following in order, the first is the cause of the intermediate cause, and the intermediate is the cause of the ultimate cause, whether the intermediate causes be several or one only. Now to take away the cause is to take away the effect. Therefore if there is no first cause among efficient causes, there will be no ultimate, nor any intermediate, cause. But if in efficient causes it is possible to go on to infinity, there will be no first efficient cause, neither will there be an ultimate effect, nor any intermediate efficient causes; all of which is plainly false. Therefore it is necessary to admit a first efficient cause, to which everyone gives the name God. 
The second way looks logically clear and ostensibly convincing. Unfortunately, for the believer, the argument contains a number of flaws which have allowed its complete demolition by philosophers David Hume (1711-1776) and Immanuel Kant (1724-1804).
Hume wrote down his critique of all major philosophical arguments for the existence of God in his posthumously published book, Dialogues Concerning Natural Religion (1779). Hume's first took to issue the fundamental assumption of the second way, that every event must have a cause. Hume contended that when we speak of cause we mean an explanation for an event. If that is so, surely at best it remains an assumption that every event must have a cause; for no one has ever provided explanations for every event that has occurred.
Second, Hume showed that what we actually observed are certain events, one following the other. We call the earlier one, the "cause", and the one immediately following it, the "effect". Now in this sequence of events, one following the other, why can't we keep on tracing the events infinitely forward or backwards?  In fact, there is no logical reason why we cannot. Take, for instance, the series of integers 1,2,3... and so on. We can see that this sequence will never end, because for every number n there is a larger number n + 1. Similarly we can trace the number backwards 1,0,-1,-2... and so on. This sequence too, has no end, because for every negative number -n there exists a larger negative number -n-1. Looking at it this way, "first cause" has as much meaning as the "largest positive number"!  As Hume rightly pointed out:
In such a chain too, or succession of objects, each part is caused by that which preceded it, and causes which succeed it. Where then is the difficulty? But the whole, you say, wants a cause. I answer that the uniting of these parts into a whole, like the uniting of several distinct counties into one kingdom, or several distinct members into one body, is performed merely by an arbitrary act of the mind, and has no influence on the nature of things. Did I show you the particular causes of each individual in a collection of twenty particles of matter, I should think it very unreasonable should you afterwards ask me what was the cause of the whole twenty. This is sufficiently explained in explaining the cause of the parts. 
What Hume is saying here is that the observable succession of events require no beginning since we can conceive of it going back to infinity. The entire chain of causes and effects also need no explaining for it is just an act of our mind trying to structure our experience.*
Furthermore, Hume argued that even if the argument is valid, i.e. that there is a first cause, it would not establish the existence of God. For one thing, why shouldn't the first cause be the universe itself rather than God. If it is argued that the universe also needs to be caused, then the same would be true for God. If one then asserts that God is "uncaused", the same assertion can be made for the universe. 
Taking over from where Hume left off, Kant continued the pounding at the flimsy foundations of the First Cause Argument. Kant pointed out that the principle of there being a cause for every event applies, as far as we know, only to the world of our sense experience. In the First Cause Argument this principle is uprooted from our world of sense experience to explain something that is suppose to transcend it. This procedure is, according to Kant, both unjustified and illegitimate, for there is no basis whatsoever to assume that the principle of causality holds when applied to the whole cosmos.
There is a further flaw in the argument. Kant pointed out that there is no rational way of actually knowing that we have reached the origins of causes and explanations. What we assume to be the first cause may just as well be due to our ignorance of the cause and explanation for it. In other words, when can we be sure we have reached the first cause? There is no method by which we can then exclaim, "Aha! This is the first Cause!" Kant concluded that these problems arise from the fundamental flaw of utilizing a principle beyond its valid range of applications. 
Modern philosophers have added to and refined the arguments of Hume and Kant. They point out that the second way, or First Cause Argument, contains an elementary logical fallacy: a quantifier reversal. An example is easy to give: suppose that every player in the basketball team has a wife; one then commits the fallacy of quantifier reversal when the fact is extended to state that the basketball team itself has a wife! This is how the first cause argument reasons: because every causal series must have a first cause, the argument claims that there must be a first cause for all such series. Stated this way, the fallacy in the argument is clear. 
Modern physics have also shown that the fundamental premise of the First Cause Argument, that every event must have a cause, to false. The phenomenon of radioactivity is one such example of an event that has no cause. While it is possible to predict the half-life of radioactive materials, i.e. how long it will take for half the original sample to decay, the exact moment when an individual atom will decay cannot be predicted. The decay of an individual atom is an example of an uncaused event. Theologians have tried to get out of this difficulty by saying that the theist denies that anything "just happens"; God directs and causes the moment of the decay, although it has no physical cause. But this defence is obviously circular. For the theologians are using the conclusion of the First Cause Argument (that God is the ultimate cause of things) to secure the validity of one of its premises (that every event must have a cause). A perfect example of the fallacy of petitio principii (begging the principle).  The second way is therefore unconvincing for the following reasons: it assumes that all sequences must be finite when we have no logical reason for believing so; we have no way of determining which cause exactly is the first cause; it commits the logical fallacy of quantifier reversal; and finally the fundamental assumption, that every event must have a cause, is shown to be untrue by modern physics. The First Cause Argument, the second way of Thomas Aquinas, has been shown the direction of the first, they are both invalid as proofs of God's existence.
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The Third Way: God, the Necessary Being
In the third way, God is defined as the Necessary Being. Aquinas:
The third way is taken from possibility and necessity, and runs thus. We find in nature things that are possible to be and not to be, since they are found to be generated, and to be corrupted, and consequently, it is possible for them to be or not to be. But it is impossible for these always to exist, for that which can not-be, at some time is not. Therefore, if everything can not-be, then at one time there was nothing in existence. Now if these were true, even now there would be nothing in existence, because that which does not exists begins to exists only through something already existing. Therefore, if at one time nothing was in existence, it is impossible for anything to have begun to exist; thus even now nothing would be in existence - which is absurd. Therefore not all beings are merely possible, but there must exist something the existence of which is necessary. But every necessary thing either has its necessity caused by another, or not. Now it is impossible to go on to infinity in necessary things which have their necessity caused by another, as has already been proved in regard to efficient causes. Therefore we cannot but admit the existence of some being having of itself its own necessity, and not receiving it from another, but rather causing in others their necessity. This all men speak of as God. 
As with the other two ways, there is a fundamental flaw in Aquinas' argument here. It is noted that the conclusion of the Cosmological Argument [c] is that there exists a being who owes its existence only to itself and nothing else. But it should be remembered that one of the basic premise of the argument is that there is no being that owes its existence to itself. This openly contradicts the final conclusion, which states that God is the being that owes its existence to itself. Thus the third way commits an elementary logical fallacy.
There has been attempts to modify the premise to say that no finite being owes its existence too itself and that since God, being infinite does owes his existence to himself. But this is basically saying that no being owes its existence to itself, except God, thus committing the fallacy of petitio principii; for the conclusion is already assumed in one of its premises. 
The word "necessary" can be used to two separate senses: the logical and the empirical. In the case where "necessary" is used in a logical sense, the proposition "God exists" can only mean that God's existence cannot be denied without contradiction. All this boils down to the assertion that "God exists" by definition. But we are now back to the Ontological Argument which we have shown to be invalid.  Now if "necessary" is used in an empirical sense, by saying that we need God as a necessary cause for everything else, we are back to the First Cause Argument. And that we have shown to be fallacious. 
Perhaps the most eloquent critique of this argument is the one by David Hume in his Dialogues. First Hume pointed out that the term "necessary existence" does not have a logical conviction of purely logical or mathematical arguments:
It is pretended that the Deity is a necessary existent being; and this necessity of his existence is attempted to explained by asserting that if we knew his whose essence or nature we should perceive to be impossible for him not to exist as for twice two not to be four. But it is evident that this can never happen, while our faculties remain the same as at present. It will still be possible for us, at any time, to conceive the non-existence of what we formally conceived to exist; nor can the mind lie under the necessity of supposing any object to remain always in being, in the same manner as we lie under a necessity of always conceiving twice two to be four. The words, therefore "necessary existence", have no meaning, or which is the same thing, none of which is consistent. 
Then Hume argued, and this is an important point, if God exists necessarily because of some unknown attribute, why should not the universe itself have such an attribute to make its existence necessary. For surely, even today, no one have the audacity to say that we know all about the universe to say otherwise:
But further, why may no material universe be the necessarily existent being, according to this pretended explication of necessity? We dare not affirm that we know all the qualities of matter; and for aught we can determine, it may contain some qualities which, were dare known, would make its non-existence appear as great a contradiction as twice two is five. I find only one argument employed to prove that the material world is not the necessarily existent being; and this argument is derived from the contingency both in the matter and the form of the world. "Any particle of matter," it is said "may be conceived to be annihilated, and any form may be conceived to be altered. Such an annihilation or alteration, therefore, is not impossible." But it seems a great partiality not to perceive that the same argument extends equally to the Deity, so far as we have any conception of him, and, that the mind can at least imagine him to be nonexistent or his attributes unalterable; and no reason can be assigned why these qualities may not belong to matter. As they are altogether unknown and inconceivable, they can never be proved incompatible with it.
We can now sum up the various problems with the Third Way: the argument, in its basic form is circular, as it assumes the conclusion in one of its premises; the term "necessary being" if it is to be used in a empirical sense boils down to the Second Way, which was already shown to be fallacious, if it is used in the logical sense it reduces to the Ontological Argument, another argument already dismissed as unsound; and finally even if we must admit a necessary existent entity why shouldn't it be the universe itself?. All in all the Cosmological Arguments fail to demonstrate the existence of God.
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The Fourth Way: God, the Absolute Being
In the fourth way, God is defined as the Absolute Being which is used as a yardstick for the measurement of all qualities:
The fourth way is taken from the gradation to be found in things. Among beings there are some more and some less good, true, noble and the like. But more or less are predicated of different things according as they resemble in their different ways something which is the maximum, as a thing is said to be hotter according as it more nearly resembles that which is hottest; so that there is something which is truest, something best, something noblest, and consequently, something which is most being, for those things that are greatest in truth are greatest in being ... Now the maximum in any genus is the cause of all in that genus, as fire, which is the maximum of heat, is the cause of all hot things, as is said in the same book. 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. 
This argument is the weakest of Aquinas' five ways. In the first place while it may be admitted that some kind of yardstick need to be applied before we can talk in terms of "more" or "less", but there is absolutely no reason why this yardstick must be absolute. This is especially true in the light of our knowledge today.
Values such as "good", "true" and "noble" actually have their assessment change across different cultures and different historical periods. For instance polygamy is considered a crime in western societies, hence is "bad" or "ignoble". Yet in Muslim countries, polygamy is not prohibited and in fact in certain circumstances, where the many menfolk are killed in war, for instance, it is actively encouraged. In ancient Confucianism, polygamy is nowhere prohibited. Thus, in these societies, polygamy is an accepted form of marital affairs.
Most social standards and norms are therefore subjective. It is simply impossible to grade the many social norms in a hierarchical sense. The absence of any clear-cut gradation in this sense refutes the Fourth Way. 
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The Fifth Way: God, the Grand Designer
In the fifth way, God is defined as the director of all actions and processes:
The fifth way is taken from the governance of the world. We see that things lack knowledge, such as natural bodies, act for an end, and this is evident from their acting always, or nearly always, in the same way, so as to obtain the best result. Hence it is plain that they achieve their end, not fortuitously, but designedly. Now whatever lacks knowledge cannot move towards an end, unless it is directed by some being endowed with knowledge and intelligence; as the arrow is directed by the archer. Therefore some intelligent being exists by whom all natural things are directed to their end; and this being we call God. 
The fifth way asserts that inanimate things and processes appear to be acting toward the best result. Hence showing that the process must be directed by an intelligent being. But what does it mean when we say that processes are tending towards the "best result"?
As an example, certainly the environmental condition of this planet seems to have been the "best result" for the things living in it. But how do we know it is not simply a fortuitous (for us) occurrence? After all, the other planets in the solar system have environments which, as far as we know, is unable to sustain any form of live. The "success rate" is one in nine , for the formation of a life sustaining environment. This rate will be even lower if we consider the moons of the planets as "candidate" worlds. Looked at this way, the life sustaining environment does look like a fortuitous occurrence. Furthermore life developed to adapt itself to the surroundings, not the other way around. It is thus impossible to make the assertion that inanimate objects or processes always seems to move towards the "best result".
The fundamental premise of the Fifth Way, that inanimate things and processes are acting towards an intelligent end, is untenable. 
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a. Aquinas' first three ways are called the Cosmological Arguments because each argument starts from there being a cosmos or world which seems to need something to account for it.
b. The term efficient cause was used by Thomas because he was influenced by Aristotelian philosophy. According to Aristotle, to have an adequate understanding of something we need to know four types of causes: the final cause, the formal cause, the material cause, and the efficient cause. As an example of this let us study a pen. It's final cause is the function of the pen, it is used for writing. It's formal cause, is its shape or form that allows it to fulfil its final cause (e.g. it's long slender shape which allows a human hand to hold it.) The material cause refers to the actual material that made it, for obviously the material must be strong enough to have a human hand holding it without bending, it's got to be impermeable to ink etc. The efficient cause is the the motion or action that begins or creates it, thus if it is a rare pen, the master craftsman in Switzerland is the efficient cause.
When we speak today about scientific causes, we are talking mainly about what Aristotle would term efficient causes. Thus I have done away with the adjective in my subsequent discussion of cause.
c. Although the term "Cosmological Argument" is used to refer to the first three ways of Aquinas, it is also used specifically to refer to the third way.
1. Benet, The Reader's Encyclopedia: p42
Livingstone, Dictionary of the Christian Church: p512
Miller, God and Reason: p43
Ross, Philosophy of Religion: p29
2. Miller, God and Reason: p44
3. Ibid: p44
4. Popkin & Stroll, Philosophy: Made Simple: p153
5. Jones, A History of Western Philosophy: II The Medieval Mind: p217
6. Lewis, Philospohy of Religion: p159
7. Jones, A History of Western Philosophy: II The Medieval Mind: p217
8. Popkin & Stroll, Philosophy: Made Simple: p153
9. Jones, A History of Western Philosophy: II The Medieval Mind: p219-220
10. Alston & Brandt, The Problems of Philosophy: p145
11. Popkin & Stroll, Philosophy: Made Simple: p154
12. Ibid: p154
13. Ross, Philosophy of Religion: p33
14. Hepburn, Christianity and Paradox: p161-165
15. Jones, A History of Western Philosophy: II The Medieval Mind: p217-218
16. Hepburn, Christianity and Paradox: p166
17. Ibid: p171
18. Ibid: p172
19. Alston & Brandt, The Problems of Philosophy: p44
20. Jones, A History of Western Philosophy: II The Medieval Mind: p218
21. Ibid: p221
22. Ibid: p218
23. Ibid: p222