While St. Anselm was Prior of the Abbey of Bec in Normandy, his fellow monks requested that he write a short treatise (Monologion) on faith and the existence of God for the purpose of their private meditation. Reflecting further on the complex line of argumentation in the Monologion, St. Anselm wondered if it would be possible to formulate one single argument for the existence of God that would appeal to all the attributes we believe of Him.
Thus Anselm wrote the Proslogion, a tract consisting of 26 chapters attempting to "prove that God really exists, that He is the supreme good needing no other and is He whom all things have need of for their being and well-being, and also to prove whatever we believe about the Divine Being". It is difficult to determine whether St. Anselm intended the treatise to be read only by believers or by non believers as well, since there is evidence to support both sides. However, since the format is such that it begins in an elevated oratorical style "rousing the mind to the contemplation of God", and concludes with "Amen", I would tend to conclude that the intent was for those who already believe. The ontological argument is found in chapters II and III.
There are actually two distinct arguments in these chapters. The first deals with proving the existence of a most perfect being, and the second proves that if this most perfect being exists, it does so necessarily. The following excerpt is from the second chapter of the Proslogion which contains the first argument:
Well then, Lord, You who give understanding to faith, grant me that I may understand, as much as You see fit, that You exist as we believe You to exist, and that You are what we believe You to be. Now we believe that You are something than which nothing greater can be thought. Or can it be that a thing of such a nature does not exist, since 'the Fool has said in his heart, there is no God' [Ps. xiii.I, lii.l]? But surely, when this same Fool hears what I am speaking about, namely, 'something than which nothing greater can be thought', he understands what he hears, and what he understands is in his mind, even if he does not understand that it actually exists. For it is one thing for an object to exist in the mind, and another thing to understand that an object actually exists. Thus, when a painter plans beforehand what he is going to execute, he has [the picture] in his mind, but he does not yet think that it actually exists because he has not yet executed it. However, when he has actually painted it, then he both has it in his mind and understands that it exists because he has now made it. Even the Fool, then, is forced to agree that 'something than which nothing greater can be thought exists in the mind, since he understands this when he hears it, and whatever is understood is in the mind. And surely 'that than which a greater cannot be thought' cannot exist in the mind alone. For if it exists solely in the mind even, it can be thought to exist in reality also, which is greater. If then 'that than which a greater cannot be thought' exists in the mind alone, this same 'that than which a greater cannot be thought' is 'that than which a greater can be thought". But this is obviously impossible. Therefore there is absolutely no doubt that 'something than which a greater cannot be thought' exists both in the mind and in reality.
The entire line of reasoning in this argument for the existence of God turns on the premise that God is 'something than which nothing greater can be thought'. St. Anselm thinks that this formula is self evident and really is implicit in the definition of what we believe to be God. However, "the Fool has said in his heart, there is no God". The following is a re-construction of St. Anselm's first argument using the text previously excerpted:
Ia. Anselm's First Argument
1. Now we believe that You are something than which nothing greater can be thought.
2. The Fool hears what I am speaking about, namely 'something than which nothing greater can be thought', he understands what he hears.
3. What he understands is in his mind (from 2).
4. And surely 'that than which a greater cannot be thought1 cannot exist in the mind alone. For if it exists solely in the mind even, it can be thought to exist in reality also, which is greater.
5. Therefore, there is absolutely no doubt that something than which a greater cannot be thought exists both in the mind and in reality.
Ib. Reconstruction of Anselm's First Argument
From Anselm's argument we can distill the following:The second proof concerns the necessary existence of God (chapters III and IV):
1. God is that than which no greater can be conceived.
2. That than which no greater can be conceived exists in reality (otherwise a greater could be conceived).
3. Therefore God exists.
And certainly this being so truly exists that it cannot be even thought not to exist. For something can be thought to exist that cannot be thought not to exist, and this is greater than that which can be thought not to exist. Hence, if 'that than which a greater cannot be thought' can be thought not to exist, then 'that than which a greater cannot be thought is not the same as 'that than which a greater cannot be thought', which is absurd. 'Something than which a greater cannot be thought' exists so truly then, that it cannot be even thought not to exist. And You, Lord our God, are this being. You exist so truly, Lord my God, that You cannot even be thought not to exist. And this is as it should be, for if some intelligence could think of something better than You, the creature would be above its creator and would judge its creator—and that is completely absurd. In fact, everything else there is, except You alone, can be thought of as not existing. You alone, then, of all things most truly exist and therefore of all things possess existence to the highest degree; for anything else does not exist as truly, and so possesses existence to a lesser degree. Why then did 'the Fool say in his heart, there is no God' [Ps. xiii. I, lii. I] when it is so evident to any rational mind that You of all things exist to the highest degree? Why indeed, unless because he was stupid and a fool?
IIa. Anselm's Second Argument
1. Something can be thought to exist that cannot be thought not to exist.
2. This is greater than that which can be thought not to exist.
3. If 'that than which a greater cannot be thought1 can be thought not to exist, then 'that than which a greater cannot be thought' is not the same as 'that than which a greater cannot be thought', which is absurd.
4. Therefore, 'something than which a greater cannot be thought' exists so truly then, that it cannot be even thought not to exist.
IIIa. Anselm's Supporting Argument
St. Anselm supports this second argument in the second half of the third chapter excerpted above:
1. If some intelligence could think of something better than You, the creature would be above its creator and would judge its creator.
2. Everything else there is, except You alone, can be thought of as not existing.
3. You alone, then, of all things most truly exist and therefore of all things possess existence to the highest degree.
IIb. Reconstruction of Anselm's Second Argument
From the above argument and its supporting argument we can distill the following:
1. God is that than which no greater is or is conceivable (from I.l.)«
2. That than which no greater is or is conceivable cannot be conceived not to exist (see argument III below).
3. Whatever cannot be conceived not to exist necessarily exists.
4. Therefore God necessarily exists.
IIIb. Reconstruction of Anselm's Supporting Argument
1. Whatever cannot be conceived not to exist is greater than what can be conceived not to exist.
2. God is that than which no greater is conceivable (from II.1. ) .
3. Therefore God cannot be conceived not to exist.
A criticism of St. Anselm's proof, entitled A Reply to the Foregoing by a Certain Writer On Behalf of the Fool, was sent to him by Gaunilo, a monk of the Abbey of Marmoutier near Tours. In total there are eight articles in his reply, seven of which offer criticisms, whereas the eighth offers praise for the remainder of the 'brilliantly argued tract' of the Proslogion. In turn, Anselm wrote a reply to these criticisms entitled A Reply to the Foregoing by the Author in Question. "Anselm's reply poses something of a problem, since it is not a systematic treatment of either the original argument in the Proslogion or Gaunilo's criticism. Rather it has a character of a series of notes". Therefore, we will look at each of Gaunilo's criticisms in his order of presentation, as well as the relevant corresponding replies of Anselm.
[1.] Gaunilo begins by simply restating Anselm's first argument as it appears in the second chapter of the Proslogion, but with one variation. Gaunilo changes Anselm's formula of 'something than which nothing greater can be thought' to 'that which is greater than everything'. Anselm responds to this in reply V as follows: "For 'that which is greater than everything1 and 'that than which a greater cannot be thought' are not equivalent for the purpose of proving the real existence of the thing spoken of. For what if someone should say that something that is greater than everything actually exists, and yet that this same being can be thought of as not existing, and that something greater than it can be thought, even if this does not exist?". There is a great difference between the two formulae; Anselm, by using 'conceived', included both possibility and actuality. Gaunilo erred by not including the possible, and thus they were referring to two different objects. Anselm here sufficiently clarifies his position and his reply to Gaunilo is well taken.
[2.] In the second article Gaunilo attacks the second premise of Anselm's first argument. Gaunilo states, "For could I not say that all kinds of unreal things, not existing in themselves in any way at all, are equally in the mind since if anyone speaks about them I understand whatever he says?". His primary objection to Anselm's premise is that something which is false or non-existent can by the very fact it is heard, exist in the mind and therefore presumably be inferred to exist in reality. There must, he concludes, be a difference in the way in which 'that than which nothing greater can be thought' exists in the mind and these non-beings exist in the mind. Anselm's reply (chapter VI) is as follows: "...I was concerned to prove something which was in doubt, and for me it was sufficient that I should first show that it was understood and existed in the mind in some way or other, leaving it to be determined subsequently whether it was in the mind alone as unreal things are, or in reality also as true things are". What Anselm clarifies is that it does not matter that false or non-existent things are conceived in the same way as a most perfect being. By that premise Anselm only wanted to prove that such a concept exists in the mind. Gaunilo's own objection proves that Anselm was successful. Therefore Gaunilo's criticism actually confirms Anselm's intended point.
[3.] Gaunilo proceeds to criticize Anselm's example of the unpainted picture: "even if it were true that there was something than which nothing greater can be thought, this thing, heard and understood, would not, however, be the same as the not-yet-made picture is in the mind of the painter". Anselm responds: "[your painstaking argument is beside the point.] For I did not propose the example of the foreknown picture because I wanted to assert that what was at issue was in the same case, but rather that so I could show that something not understood as existing exists in the mind". I think Anselm did not directly respond to the point in this objection and he failed to see the importance of Gaunilo's criticism. Anselm agreed that there is a great difference between the painter having a clear and distinct understanding of what he is about to complete, and the finished painting. However, the distinction was between existence in the mind and necessary instantiated existence. For example, it is possible to have the concept of an existent round square. By combining the concepts of existence, roundness and squareness it is possible to conceive of such an object as existent by virtue of its very nature or definition. It does not necessarily follow, though, that this object exists in reality, for we know that it is an impossibility, even though "existent" belongs to its definition. So it is the same with God. We can conceive of 'something than which nothing greater can be conceived' as truly existing in our minds, but it does not follow that such a being, even if necessarily conceived as existent is necessarily real. Gaunilo wins this point.
[4.] The next criticism reinforces the previous one: Gaunilo states that not even the idea can be known. "If I heard something said about a man who was completely unknown to me so that I did not even know whether he existed, I could nevertheless think about him in his very reality as a man by means of that specific or generic notion by which I know what a man is or men are. However, it could happen that, because of a falsehood on the part of the speaker, the man I thought of did not actually exist, although I thought of him nevertheless as a truly existing object". He then states that God, although one can hear about him, cannot even be known to this limited degree because his 'genus and species' are not known. It follows then that since God cannot even be understood, only the individual words used to describe Him, there is no idea of Him to which one might ascribe real existence. Anselm responds in the first article that his strongest argument is an appeal to Gaunilo's faith. This in fact is the weakest argument. He then continues:
[5.] "If something that cannot even be thought in the true and real sense must be said to exist in the mind, then I do not deny that this also exists in my mind in the same way. But since from this one cannot in any way conclude that it exists also in reality, I certainly do not yet concede that it actually exists, until this is proved to me by an indubitable argument". Gaunilo explains that understanding something named does not mean it can be inferred to exist. Anselm responds in articles I and II: "I insist, however, that simply if it can be thought it is necessary that it exists. In the second article Anselm reiterates his original sound major argument that to think of 'that than which nothing greater can be conceived' as not existing is contradictory. But it is clear that he and Gaunilo are arguing at cross-purposes.
[6.] This is Gaunilo's famous example of the Lost Island. Suppose there is a most beautiful island somewhere in the ocean that is better than all others in reality as well as conceptually. Because it is better to exist in reality than only in the mind, it logically follows that this most perfect island of which there is no greater necessarily exists in reality. He says that this is absolutely foolish. Anselm replies that his formula only applies to a Most Perfect Being, since the inference to a Most Perfect Being is based on pure ontological excellence, not on excellence within a particular genus.
[7.] Gaunilo's final criticism maintains that God can be thought not to exist. He states, "When have I said that there truly existed some being that is 'greater than everything', such that from this it could be proved to me that this same being really existed to such a degree that it could not be thought not to exist?". Anselm replies that it would be better to say that God cannot be understood, rather than he cannot be thought not to exist. Anselm's reply is beside the point. He ought to have said that a conception of a God who might not exist was a defective conception and not the correct of a Most Perfect Being.