LECTURE #4: Ibn-Sina, Maimonides and Aquinas

I. Ibn Sina


In Ibn Sina, it is especially clear that the distinction between essence and existence (in contingent beings) is supposed to be a real, and not just a logical or mental, distinction. Ibn Sina describes 'existence' as a kind of "accident" that is super-added to the essence of a thing, thereby bringing one thing of that kind or essence into real existence. At the same time, Ibn Sina recognizes that is is not quite appropriate to say that existence is a "property" of a thing -- as though, in addition to having two legs and being warm-blooded, I have the additional property of existing. Existence is a special sort of accident -- one that does not characterize things, but which constitutes their being.

As I explained last time, al-Farabi and ibn Sina are "realists", in the sense that they believe that essences and accidents (properties) are real constituents of the world, not things that are merely invented by us and projected on the world through language or thought. Therefore, to learn about the essence of a thing, we must engage in scientific and metaphysical investigation - it is not enough merely to introspect and examine our own ideas. We do not discover that God's essence is identical to His existence by examining our subjective idea or conception of God. Instead, the cosmological argument leads us to the conclusion that there must exist something whose essence is identical to its existence, and further investigation enables us to recognize that this something has all the characteristics we associate with the idea of God.

The American philosopher Saul Kripke of Princeton has revived realism about essences, through his book, Naming and Necessity. Take the example of water. Before the development of chemistry, people conceived of water as a transparent, odorless liquid that freezes at 0 C, etc. However, none of these characteristics constitute the essence of water. Instead, we no know that the essence of water is (and always has been) something like: consisting of molecules of H2O. Similarly, our idea of the person Homer associates him with the authorship of the Iliad and the Odyssey. However, Homer's essence has nothing to do with creating these epics. Homer could have existed without composing any poetry. His real essence consists of something like: human being, child of x and y (where x and y are Homer's actual parents).

According to the realist, the essence, the what-it-is of a thing, is a real component of facts or situations in the world. Similarly, the existence of something is a real component of the world: an act or event or happening by which some particular thing comes to be. For ordinary objects, of the kind that we encounter in sense experience, essence and existence are distinct. However, the fact that there must be an uncaused first cause forces us (according to al-Farabi and ibn Sina) to recognize one case in which they are identical.


It is in the area of rationale that Ibn Sina makes his most significant contribution. Ibn Sina introduces into the tradition a new argument for the necessity of a first cause: what I shall call the "aggregative argument". This argument is developed in ibn Sina's book, al-Najat. Let's assume that every contingent being has a cause. Consider the aggregate of all contingent beings -- that huge entity that contains all of the contingent beings of the world as parts, and that contains nothing else. Let's call this aggregate entity C. Is C itself a contingent being? Yes, it clearly is. Remember that C is the aggregate of all the actually existing contingent things. If any part of C ceases to exist, C itself ceases to exist.

Here's an analogy -- let Y be the aggregate of all the members of the current roster of the NY Yankees. If a member of Y drops off the Yankees, Y will still exist, but Y will no longer constitute the aggregate of current Yankees. If one of the current members of the Yankees dies, then Y ceases to exist, even though the NY Yankees (as an organization) continues to exist. Similarly, even if it were necessary that there be some universe, it doesn't follow that C exists necessarily. Since there are parts of C that could fail to exist (in fact, this is true of every part of C), C itself could fail to exist. So, C is a contingent being. So, C must have a cause.

The cause of C cannot be a part of C -- it cannot even overlap with C. Causes and effects are completely separate. Thus, the cause of C, call it N, cannot be a contingent being. If N were contingent, then it would be a part of C, since C was defined to be the aggregate of all contingent beings. Consequently, N must be a necessary being. Notice that this argument does not depend at all on ruling out the possibility of an infinite regress!

Ibn Sina goes on to argue that there must be an absolutely first cause. Unlike al-Farabi,Ibn Sina thought that there were many necessary beings. He followed neo-Platonists like Philo in thinking that a number of necessary beings "emanated" from God. These might include such things as numbers and ethical universals (justice, friendship, etc.) Ibn Sina argues that any being, necessary or contingent, for which essence and existence were distinct, must be caused. He uses the standard no-infinite-regress argument to conclude that there must be a first cause of all such beings, and this being must be such that its existence and its essence are identical.


Ibn Sina very explicitly accepts a necessitarian model of causation. Everything flows from God by a chain of necessary implications. Contingent beings are those that do not exist at all times. The existence of the universe is a necessary consequence of God's being, not an act of free choice.


Like al-Farabi, ibn Sina assumes a synchronic model of causation. Something must be explaining NOW the present existence of any contingent being. However, it is not clear that his argument depends on this model. Even if we thought of causation as diachronic, so the First Cause would act in the past to set in place a chain of successive beings, the argument seems to go through.

Like al-Farabi, ibn Sina accepts Aristotle's argument for the eternity of the world of time and motion. An alternative tradition of philosophical theology, the Kalam tradition, rejected Aristotle's philosophy and insisted that the world was created in time, at a particular point in the past. We will look at the legacy of the Kalam tradition next week, when we examine Kant's first antinomy and contemporary arguments for the Big Bang.


Ibn Sina starts with the fact that God is absolutely necessary, independent, and uncaused. These facts entail that God is absolutely simple -- that there is no composition, no putting-together of distinct things, in God's being. Consequently, there can be only one God, since there would be nothing that could differentiate two absolutely simple beings. Since God is absolutely necessary and uncaused, He can have no accidents, no aspect of potentiality. This entails that He is immutable. Finally, ibn Sina concludes that God is absolute Perfection, since He is the cause of all things, and no effect can exceed its cause. That is, there can be no excellence in the cause that is not present (in some way) in the cause.

The God of al-Farabi and ibn Sina is radically Other than ourselves. It seems that this conception of God belies Karl Barth's worry that natural theology necessarily leads to a blurring of the difference between God and us. It is hard to imagine a conception of God that involves a more radical difference between God and all other things. It is admitted by a-F and i-S that the nature of God is utterly incomprehensible by us. There is no analogy that can help to illuminate for us what it would be like for a thing to have an essence that is identical to its existence. Nothing else is similar to God in this respect. Every inference about that we can draw from this starting-point merely highlights the gulf between us and God. God is unique in every respect, radically simple where we are complex and compounded, immutable where we are ever-changing.

What sense can we make of this radically incomprehensible hypothesis? We cannot imagine what it means, but we can deduce certain consequences, relying on the austere universality of pure logic. First of all, this hypothesis means that for God there is no distinction between possibility and actuality. Whatever God is possibly, He is in actuality. Whatever He is in actuality, He had to be: He could not have been otherwise. In particular, God exists necessarily: He could not have failed to exist, since for God to fail to exist would involve a separation of His existence from His essence, which is impossible, since they are one and the same thing.

According to ibn Sina, God is not the only necessary being. There are other beings that God necessarily brings into existence. Nonetheless, God is unique, since only God is necessary in and of itself. Other necessary beings exist because of God -- we can explain their necessary existence by reference to their being caused to exist by God. God's existence is absolutely independent and uncaused. There is no priority of possibility over actuality, so we cannot properly ask the question: why is the possibility of a divine being realized in fact?

Since ibn Sina is a realist, this argument is quite different from the so-called "ontological argument" of Anselm of Canterbury or Rene Descartes. In the ontological argument, we define God as the being whose essence is (or includes) existence. We then conclude that God, so defined, must exist, since the essence cannot be separated from actual existence. The ontological argument starts with our own conception of God, or with a verbal definition that we introduce by stipulation. Unlike ibn Sina's argument, it does not include any demonstration that a being really exists whose essence is identical to its existence. We have no reason, apart from the cosmological argument, that there really is an essence that includes existence.


Moses Maimonides was the greatest Jewish philosopher of the medieval period. While living in Spain, Maimonides was exposed to the Aristotelian philosophy of al-Farabi, ibn Sina and others. Unlike the Arab Aristotelians, Maimonides was convinced that the world was created at a particular point in the past. He rejected the thesis that time and motion are eternal. However, unlike the theologians of the Kalam school (such as al-Ghazali), Maimonides did not reject Aristotle's philosophy. In particular, he sought to modify Aristotle's cosmological argument in such a way as to make it compatible with creation in time.

Maimonides lists 26 basic principles of Aristotle's philosophy, and he accepts all but the last (the eternity of the world). Maimonides thought that the eternity of the universe was compatible with the Bible and the Jewish faith (since he was willing to interpret Genesis figuratively), but he believed that there were good scientific and philosophical reasons for believing in a creation in time.


Maimonides gives four versions of the cosmological argument. The third proof is most interesting -- the proof of necessary being. Maimonides seeks to prove that it is impossible for every being in the world to be contingent (to be capable of non-being). He argues that if every being were contingent, there would have been a time in the past during which nothing existed at all. His argument seems to set up a dilemma: either the whole world began to be at some point in the past, or the past is infinitely long. If the whole world began to exist at some point in the past (as Maimonides himself believes), then there must have been an eternal cause that brought the world into being at that time. (Maimonides seems to be assuming that an eternal being would be a necessary being.) Alternatively, if the past is infinitely long, and every being is contingent, then there would have been some time at which everything ceased to exist simultaneously.

Maimonides accepted that Aristotelian principle that there can be only finitely many things in the world. He also seems to be assuming something like this: if a being exists only contingently, there is a finite probability that it cease to exist at this very moment. If the existence or non-existence of one thing is independent of another, then there is always a finite probability that everything should cease to exist at this very moment. Over a period of infinite time, every possibility that has a finite probability must eventually occur. Consequently, at some point in the past, everything would have ceased to exist.

However, if there is once nothingness, there will always be nothingness thenceforth, since nothing causes nothing. Consequently, nothing would now exist, which is obviously false.

Notice that this argument gives us some reason for thinking that an infinite causal regress is impossible. The argument turns on the absolute length of an infinite past, and on some intuitive and informal probabilistic reasoning. Perhaps the weakest point is the assumption that the continued-existence or annihilation of each thing is independent of that of every other thing. Perhaps everything is contingent, but it is impossible for everything to cease to exist simultaneously, since the annihilation of some things prevents the annihilation of others.


Maimonides explicitly rejects the view that causes necessitate their effects. He follows the lead of some of the Kalam theologians, introducing the idea of a "determinative". A determinative makes its effect possible, but it does not necessitate that effect. An act of free will is the paradigm case of a determinative. If I am hungry, and there are identical supplies of food to my left and to my right, my will is capable of moving me to one or the other, despite the fact that nothing about me or my situation necessitated my moving one way or the other. In a similar way, God could have chosen not to create, or He could have chosen to create any of a vast number of possible universes, all of which would have expressed His benevolence and wisdom. Nothing necessitated His creating this one.

Maimonides emphasizes the absolute contingency of the created world. He points out that Aristotle cannot explain why there are stars, or why the stars are arranged in the very specific and haphazard way they are in fact arranged. Surely many other arrangements would have been possible and would have suited God's purposes equally well. The best explanation for the actual arrangement is a free choice on God's part.

Maimonides saw a connection between contingency and creation in time. He believed that if God caused the universe eternally (as Aristotle thought), then the universe would have to be a necessary emanation from God's nature, with no room for free choice or real contingency. Nothing could have been different than it actually is. However, the world is obviously contingent, the product of free choices, so it must have been created in time. However, I cannot see why a free, undetermined choice in eternity, resulting in an eternal but radically contingent world, should be impossible.

Thomas Aquinas

Like Maimonides, Aquinas wanted to combine Aristotelian philosophy with the biblical idea of the creation of the world in time. He very explicitly sets up the dilemma (pp. 31-32): either the world began to exist at some point in the past, or it is eternal. If the world began to exist, then it is obvious that there must be an eternal first cause that brought it into existence. If the world is eternal, then we can turn to Aristotle's arguments and still demonstrate that an uncaused first cause must exist.

Aquinas has three versions of the cosmological argument: from motion, from efficient cause (which seems to be the argument from the actualization of potential), and from contingent existence. These arguments follow closely the patterns set by Aristotle and al-Farabi. In particular, Aquinas relies heavily on the rejection of infinite regresses, and his reasons for this rejection are simply those of Plato and Aristotle.


Aquinas' greatest originality lies in the closure of the argument: the transition from the first cause to a rich theology. He starts as does al-Farabi: with a being who exists necessarily. He goes into great detail in arguing that a being who exists necessarily must be one whose essence is identical to His existence. He also goes to somewhat greater lengths than earlier thinkers in drawing out further implications of this starting point.

Aquinas argues first that a first cause must contain absolutely no potentiality. Otherwise, there would have to be another being that actualizes the potential of the First Cause, which is impossible (by definition). Consequently, God is absolutely simple -- He has no parts, is not in any sense the product of the combination of distinct elements. If He were composite in any way, we would have to find a cause of the composition, the putting-together of these elements.

Since God has no potentiality, He has no accidents. This means that there is no distinction God has no property other than His essence. Moreover, since God is simple, He cannot be composed of both matter and form. Since He must have a form or essence (since everything, God included, must be a something-or-other), God must be immaterial. Since God has no matter and no accidents, there is nothing to distinguish God from His own essence. Consequently, God is His essence.

Again, since God is absolutely simple, His existence cannot be the result of the combination of an act of existence with His essence. There can be no possibility of God's existence that is prior to His actually existing, since if there were, we would need a cause to explain the actualizing of that possibility. If God's essence and existence were two different things, then one would depend on the other. If God's existence depended on His essence, then His existence would have a cause, which is impossible. But, God's essence cannot depend on His existence, since this would make His essence accidental, something contingently added to His existence. Thus, the only possibility that remains is that His essence is identical to His existence.