Thomas Aquinas and the Five WaysSt. 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.
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 MoverIn 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.
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 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:
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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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:
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:
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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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 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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