Proof of the Contingent and the Necessary
Prof. Dr. Ahmad Ahmadi
Before taking up this proof it may be necessary to consider the problem of causality as a prerequisite; for understanding the argument of contingency and necessity is difficult without understanding causality.
Principle of Causality
One of the most important problems which in all philosophical schools has given rise to a great amount of debating is the question of cause and effect, or the principle of causality. It is this principle that is said to be the basis of all man's intellectual and practical efforts, for we can see that the work of scientists and their endeavours are aimed at either finding causes and effects, or determining how they correlate. A chemist, for example, who undertakes a chemical experiment, hopes to discover the real properties of the substance he is working on, or to find the effects that can be related to an element under experiment. Likewise, a historian looking into the events responsible for the outbreak of the two World Wars is after the relationship of these historical phenomena with the causes and reasons which might have led to them.
In the course of his search, the scientist wishes not only to see that two things have some kind of relationship, but he also wants to discover which one is the cause or originator and which is the effect or product. This is also true of human actions; for whatever man does is either voluntary, that is, according to his will and desire, or involuntary, that is, natural and forced. If it is voluntary, then we may say that the act is the effect of a thinker's thought, since it is he who, through thought, deliberation, and after weighing various alternatives, has made his choice, whereas involuntary actions are those which come about as a result of some bodily or natural causes. It is impossible to conceive of a thing as coming into existence without a cause and by itself. Therefore, we may say that for a thing to exist there has to be something which is a cause of its existence. Thus, to define `cause', we may say that a cause is something from which some new thing called `effect' is born. We may add that as long as the cause (that is whatever is required for origination of the effect) is not fully realized, the effect will not come to exist. On the other hand, if an effect comes into being, we can immediately assert that a cause has pre existed it. From what we have said we can conclude that causality is a relation between two beings to the effect that the existence of one is a prerequisite for the existence of the other. In this interdependence, the effect is sequential and dependent on the cause. The principle of causality can be concluded from the following analysis:
There can be only two basic explanations regarding existence of a phenomenon:
a) It comes into being accidentally or by chance; 
b) It is the effect of a cause on which it depends.
If the first assumption is accepted, then we have to accept also, firstly, that there is no provable connection between things, or the effects produced by them. For example, there is no way of proving that the fruit is born of the tree, or a man's actions are attributable to him, and, as a result, one can deny one's acts to be one's own and readily regard them as being without causes or as being accidental.
Secondly, we must accept that there can be no valid scientific law; for every law of science is based upon the principle of causality and on the notion that every phenomenon is the product of a cause and dependent upon it.
Thirdly, it should also be accepted that no event can be predictable, that is to say we must allow that many things are probable to occur at every moment, for nothing is a precondition for any other thing.
It is obvious that the above three conclusions are invalid, and the assumption that chance and accident is responsible for emergence of phenomena cannot adequately explain the problem. Thus, we have no alternative except to accept the second assumption, which entails the acceptance of the causal relationship between things. The principle of causality was looked upon by almost all philosophers except the empiricists as a reasonable way of explaining all phenomena, whether social or natural. It was the English empiricist David Hume who rejected the principle of causality, claiming that causal relationship is not what its exponents assert as an objective reality, but a subjective notion based on conjunction conceived by the mind between impressions. But it has been proven that such an interpretation of causality leads to pure solipsism and complete denial of the external world. Incredibility of such a view is self evident.  On the other hand, some Islamic philosophers hold that ideas such as causality and the like are intuitively obtained by the self through its direct knowledge of itself (`ilm huduri). So, it is claimed that not only the ideas of causality and substance, but all the basics of human knowledge are rooted in ‘ilm huduri . 
On the basis of what has been said, it is evident that causality is an objective, real and external relation, not a subjective formulation as empiricists claim it to be. Therefore, whenever a complete cause a cause which possesses all conditions necessary and sufficient for existence of the effect is realized in the external world, it is necessarily followed by the effect. Likewise, whenever we observe the effect as existing, we can conclude that its cause has already been existing. 
Types of Causes
Causes are of different types; amongst them are: complete and incomplete, simple and compound, real and preparing, contiguous and distant, internal (matter and form) and external.
A complete cause is one which has in itself all necessary and sufficient conditions for the realization of the effect. On the other hand, the incomplete cause is one which lacks some of the necessary factors for realization of the effect; it is not sufficient for creating an effect. In short, the absence of cause (whether complete or incomplete) implies the absence of the effect.
A compound cause, is one which has parts. These parts may be either material or rational (formal); for instance, the idea of species is a compound of the ideas of genus and differentia. A real cause is like causality of the self in relation to its own states such as knowledge. An example of the preparing cause is situation of a moving body at preceding positions in space for arriving at a successive position.
A contiguous cause directly acts to produce an effect, as, for example, the movement of a hammer is the direct cause of driving in of a nail. But the distant cause is an indirect cause, such as the movement of one's hand by exercise of one's will; the will acts on nerves, the nerves on muscles and muscles move to cause the movement of the hand.
Material, formal, efficient and final causes can be explained by the example of a chair: wood is its material cause; the shape its formal cause; the carpenter its efficient cause, and the purpose for which it is made, namely, to sit on it, is its final cause. The first two of these are called `internal' causes, and the latter two are termed `existential' causes (al `illah al wujudiyyah).
Relation between Cause and Effect
Now that we understand the meaning of causality, we can enter into a more elaborate examination of the relation between cause and effect. Following are some important points.
1. Relation of Necessity
As was pointed out, once the complete cause is realized, the effect will also be realized; and whenever we come across an effect, we reach the necessary conclusion that there must have been a cause previously realized. Therefore, the relation of necessity between cause and effect is a real one; that is, it is a kind of relation which cannot possibly be changed into some other kind of relation. In other words, the relation between cause and effect is like the relation between natural integers. Just as it is necessary for number 4, for example, to occur between the numbers 3 and 5, or otherwise the series is rationally distorted, so also the necessary relation between every cause and effect is that of succession in which the position of any element in the series cannot be changed. According to what has been said, any doubt about the existence of necessary relation between cause and effect is, in fact, a doubt about the validity of the principle of causality itself.
2. Relation of Homogeneity
A state of homogeneity exists between cause and effect; that is to say, every effect has some kind of agreement and consistency with the cause which has produced it, which it does not have with all other things. Or else, anything could be a cause for anything else, or anything be an effect of anything else, whereas it is not so. Certain effects result from certain causes only, and this relation, which is a necessary one, can in no way be upset or altered. And this is what we call the relation of homogeneity between cause and effect. As a matter of fact, negation of homogeneity is, in fact, negation of the whole system of necessity of causality and admission of disorder and chaos in the scheme of the universe.
3. Impossibility of Endurance of Effect without Presence of Complete Cause
Commonly we are inclined to think that it is possible for the effect to survive the cause. In this regard example is given of enduring of a building after its builder or enduring of artifacts after a craftsman has made them. But if the meaning of cause and effect and their relations is really understood and the true concept of causality is known, then it will be seen that the effect is like the shadow or reflection of the cause, so that it can never be separated from it, as it is not possible to separate the shadow from its object. Such examples, which are erroneously forwarded, concern types of incomplete cause, not the complete cause. In the case of a building's survival after its builder, the existence of the builder alone is not the complete cause for the building's endurance; rather, it is the quality and composition of the construction materials (along with other factors) which constitute the complete cause for the building to endure.
Having dealt with the nature of causality, now we shall return to our main subject: the argument of contingency and necessity, a theological argument to prove the existence of God by demonstrating dependence of all contingent things on the Necessary Being.
If we consider the nature of a thing, we cannot say of it that it should necessarily have existed or not existed. To our rational mind its relation to existence or non existence is neutral. If the object exists in the external world, then there must have been a cause which has brought it into existence. For, on the basis of the principle of causality nothing can come into existence without a cause, and that for every phenomenon or event there has to be a cause. A being which has the characteristic that its existence depends on that of another, is called a contingent being (mumkin al wujud).
A Necessary Being (wajib al wujud), on the other hand, is such that no reason can be found for attributing its existence to that of another being. Therefore, necessity, when applied to existence, means independence of existence; it is characteristic of a Self existing Being, which is self sufficient and independent of other things for Its existence.
Having briefly explained these two terms, let us now see how we can employ the contingent and necessary argument to prove the existence of God. There is no doubt that some things in the universe come into being which did not exist before.
We can see many examples of this in nature, such as blossoming of trees in spring after falling of their leaves and flowers during autumn, passing of nights and coming of days, ending of spring and beginning of fall, youth is followed by old age and old age by death, and so on. Right at this moment, I perceive sounds, flavours, sensations of touch which did not exist few moments ago (acquired knowledge or `ilm husuli).
At this very moment I have a feeling of love and affection for the people around me, and I am moreover conscious of such feelings; while only a few moments ago I did not have such sentiments, nor was I aware of them. Now, I have awareness of my own self and I am aware of this awareness (`ilm huduri).
All these things which did not exist before and are now existent are called phenomena. It is self evident that no phenomenon can be without a cause. This means that the existence of every phenomenon is entirely dependent on that of another, and, therefore, it is contingent. Now the question arises whether or not the cause of a phenomenon or any contingent being can also be a contingent being dependent on other beings, or if it has to be self existent, independent, or, what is called, a Necessary, Self existing Being.
In answer it may be said that both cases are possible; that is, the cause may be either contingent or necessary. Now, if this cause (the producer of the contingent effect), is itself a necessary being, then our claim of necessary cause is proven. But if it is contingent, then we are faced with two alternatives:
1. either it is the effect of another contingent cause, which in turn is itself the effect of another cause, and so forth till infinity (a linear or infinite series);
2. or the chain of cause and effect is not linear but circular; that is the cause at the beginning end of the chain of causation is itself the last effect at the end of the chain.
Therefore, in the case we accept the second alternative, that is, if we consider all causes in the chain of causation as being contingent, there are only two plausible hypotheses: (i) an infinite series (ii) or a vicious circle. Accordingly, the existence of the Necessary Being cannot be proved unless we follow Ibn Sina in showing that an infinite series or a vicious circle of causation is absurd and that the chain of causation should necessarily end in the Necessary Being. In other words, we have to show the impossibility of a linear or circular chain of causation which does not end in the Necessary Being.
Falsity of Circular and Infinite Series in Causation
The invalidity of circular series can be proven in this manner. There can be two or more elements in the circle of causation. If there are only two elements A and B, we may represent the causal relationship by writing. If there are more than two elements, for instance A, B, C and D, we may represent the causal relationship in this way: . In this case, A is the cause of B, B is the cause of C, C is the cause of D, and D is again the cause of A. Both the cases are, however, invalid, since it entails that A should be existent and nonexistent at the same time: it should exist to create B and not exist to be created by B. This is self contradictory and stands refuted. Also, the circle with more than two elements implies a succession of several effects without a cause, which is also seen to be invalid.
As to the infinite series, there are many reasons for considering it invalid. We shall discuss two of them here.
a) If we consider a chain of causes and effects whose last link is an effect which has not yet become a cause of another thing for example, a slight movement of the hand, which we consider as an effect of preceding causes, but it is not yet a cause for another effect we shall see that every preceding link of this chain, which precedes this last effect, must at the same time be a cause for its next link and an effect for its previous link, and so on. Thus, every link of this chain is at the same time a cause and an effect, and we know that if a link in the chain is the cause of its succeeding link that does not excuse it from the need of a cause preceding it. Therefore, every link in the series is an effect dependent on a cause.
Now supposing that this chain is infinite and there is no First Cause, it implies an infinite number of middles with no sides , which is of course impossible.
For example, in the chain we take A to be the last effect (the slight motion of the hand which has not yet become a cause for another thing). 
The link B (the muscles, in our example) is the cause for A and is also the effect of C or the effect of nerves). Link C is the effect of D (the effect of will, in our example). Thus every one of the links of the chain is both a cause and an effect at the same time. As has been argued, though every link is the cause for the following link, it does not mean that it can itself dispense with a cause preceding it. Therefore, every link is necessarily an effect, too. So we may overlook the causation of all links and illustrate them thus: ; this means that we have an endless chain of effects without coming across anything which is a cause without being an effect; as we have already shown, it is impossible.
b) We said that the existence of an effect or phenomenon is dependent, not self existent, or in other words, it is a contingent being or an intermediary existent. Now the supposition that there is an endless chain of contingent beings, implies an infinite chain of dependent and intermediary beings which do not depend for their existence on an independent, self existent being, and this is impossible.
From what we have said, we can draw this conclusion: first, that we perceive the existence of phenomenon or the contingent being either through our acquired knowledge or through introspection (`ilm huduri); i.e. we perceive the external world through acquired knowledge (`ilm husuli) and the inner states of the self through introspection (`ilm huduri); second, that every contingent being is dependent on a cause, otherwise it cannot possibly be existent; third, the chain of contingent beings and causes must inevitably originate from the First Cause, the Self existent Being, that is, God.
Khwajah Nasir al Din al Tusi has formulated the argument of the Necessary Being in this statement:
Which means, if we accept the cause of all existence as the existence of the Necessary Being, we have reached the desirable conclusion; otherwise that is, if we do not accept the Necessary Being as the first cause the existence of the Necessary Being is necessitated by the impossibility of a causal circle and a causal linear series.
. By `chance' here we mean occurrence of a phenomenon without cause. The word is also used for two other meanings: i) occurrence of a phenomenon through the agency of an unusual cause; ii) an unexpected outcome of an action which was not the premeditated goal of the doer.
. See Paul Foulquie's Treatise on Metaphysics; Persian trans. by Y. Mahdawi. See also Comparative Ideology published by Dar Rah a Haqq Organization.
. For more details see The Principles and Method of the Philosophy of Realism by `Allamah Tabataba'i, footnotes by Murtada Mutahhari, vol. 2.
. It should be noted here that causal precedence is not the same thing as temporal precedence. For example, when we move a pen, while writing, we usually say that the hand moves first, and then the pen follows the movement of the hand, or that the motion of the hand precedes that of the pen. However, they both move simultaneously. This type of precedence is called `causal precedence'.
. Ibn Sina, and other Muslim philosophers after him, termed this argument as the "middle and side argument"
. Here `c' stands for cause and `e' for effect.