ANSELM ON GOD'S EXISTENCE

PROSLOGION
Chapter 1: Encouraging the Mind to Contemplate God
Come on now little man, get away from your worldly occupations for a while, escape from your tumultuous thoughts. Lay aside your burdensome cares and put off your laborious exertions. Give yourself over to God for a little while, and rest for a while in Him. Enter into the cell of your mind, shut out everything except God and whatever helps you to seek Him once the door is shut. Speak now, my heart, and say to God, "I seek your face; your face, Lord, I seek."
Come on then, my Lord God, teach my heart where and how to seek you, where and how to find you. Lord, if you are not here, where shall I find you? If, however, you are everywhere, why do I not see you here? But certainly you dwell in inaccessible light. And where is that inaccessible light? Or how do I reach it? Or who will lead me to it and into it, so that I can see you in it? And then by what signs, under what face shall I seek you? I have never seen you, my Lord God, or known your face. What shall I do, Highest Lord, what shall this exile do, banished far from you as he is? What should your servant do, desperate as he is for your love yet cast away from your face? He longs to see you, and yet your face is too far away from him. He wants to come to you, and yet your dwelling place is unreachable. He yearns to discover you, and he does not know where you are. He craves to seek you, and does not know how to recognize you. Lord, you are my Lord and my God, and I have never seen you. You have made me and nurtured me, given me every good thing I have ever received, and I still do not know you. I was created for the purpose of seeing you, and I still have not done the thing I was made to do.
Oh, how miserable man's lot is when he has lost what he was made for! Oh how hard and dire was that downfall! Alas, what did he lose and what did he find? What was taken away and what remains? He has lost beatitude for which he was made, and he has found misery for which he was not made. That without which he cannot be happy has been taken away, and that remains which in itself can only make him miserable. Back then man ate the bread of angels for which he now hungers, and now he eats the bread of griefs which he did not even know back then. Alas for the common grief of man, the universal lamentation of Adam's sons! He belched in his satiety, while we sigh in our want. He was rich, we are beggars. He happily possessed and miserably abandoned, we unhappily lack and miserably desire, yet alas, we remain empty. Why, since it would have been easy for him, did he not keep what we so disastrously lack? Why did he deprive us of light, and cover us with darkness instead? Why did he take life away from us and inflict death instead? From what have we poor wretches been expelled, and toward what are we being driven? From what have we been cast down, in what buried? From our fatherland into exile, from the vision of God into blindness. From the happiness of immortality into the bitterness and horror of death. What a miserable transformation! From so much good into so much evil! A heavy injury, a heavy, heavy grief.
I have come to you as a poor man to a rich one, as a poor rich to a merciful giver. May I not return empty and rejected! And if "I sigh before I eat" (Job 3:4), once I have sighed give me something to eat. Lord, turned in (incurvatus) as I am I can only look down, so raise me up so that I can look up. "My iniquities heaped on my head" cover me over and weigh me down "like a heavy load" (Ps. 37:5). Dig me out and set me free before "the pit" created by them "shuts its jaws over me" (Ps. 67:16).Let me see your light, even if I see it from afar or from the depths. Teach me to seek you, and reveal yourself to this seeker. For I cannot seek you unless you teach me how, nor can I find you unless you show yourself to me. Let me seek you in desiring you, and desire you in seeking you. Let me find you in loving you and love you in finding you.
I acknowledge, Lord, and I give thanks that you have created in me this your image, so that I can remember you, think about you and love you. But it is so worn away by sins, so smudged over by the smoke of sins, that it cannot do what it was created to do unless you renew and reform it. I do not even try, Lord, to rise up to your heights, because my intellect does not measure up to that task; but I do want to understand in some small measure your truth, which my heart believes in and loved. Nor do I seek to understand so that I can believe, but rather I believe so that I can understand. For I believe this too, that "unless I believe I shall not understand" (Isa. 7:9).
Chapter 2: That God Really Exists
Therefore, Lord, you who give knowledge of the faith, give me as much knowledge as you know to be fitting for me, because you are as we believe and that which we believe. And indeed we believe you are something greater than which cannot be thought. Or is there no such kind of thing, for "the fool said in his heart, 'there is no God'" (Ps. 13:1, 52:1)? But certainly that same fool, having heard what I just said, "something greater than which cannot be thought," understands what he heard, and what he understands is in his thought, even if he does not think it exists. For it is one thing for something to exist in a person's thought and quite another for the person to think that thing exists. For when a painter thinks ahead to what he will paint, he has that picture in his thought, but he does not yet think it exists, because he has not done it yet. Once he has painted it he has it in his thought and thinks it exists because he has done it. Thus even the fool is compelled to grant that something greater than which cannot be thought exists in thought, because he understands what he hears, and whatever is understood exists in thought. And certainly that greater than which cannot be understood cannot exist only in thought, for if it exists only in thought it could also be thought of as existing in reality as well, which is greater. If, therefore, that than which greater cannot be thought exists in thought alone, then that than which greater cannot be thought turns out to be that than which something greater actually can be thought, but that is obviously impossible. Therefore something than which greater cannot be thought undoubtedly exists both in thought and in reality.
Chapter 3: That God Cannot be Thought Not to Exist
In fact, it so undoubtedly exists that it cannot be thought of as not existing. For one can think there exists something that cannot be thought of as not existing, and that would be greater than something which can be thought of as not existing. For if that greater than which cannot be thought can be thought of as not existing, then that greater than which cannot be thought is not that greater than which cannot be thought, which does not make sense. Thus that than which nothing can be thought so undoubtedly exists that it cannot even be thought of as not existing.
And you, Lord God, are this being. You exist so undoubtedly, my Lord God, that you cannot even be thought of as not existing. And deservedly, for if some mind could think of something greater than you, that creature would rise above the creator and could pass judgment on the creator, which is absurd. And indeed whatever exists except you alone can be thought of as not existing. You alone of all things most truly exists and thus enjoy existence to the fullest degree of all things, because nothing else exists so undoubtedly, and thus everything else enjoys being in a lesser degree. Why therefore did the fool say in his heart "there is no God," since it is so evident to any rational mind that you above all things exist? Why indeed, except precisely because he is stupid and foolish?
Chapter 4: How the Fool Managed to Say in His Heart That Which Cannot be Thought
How in the world could he have said in his heart what he could not think? Or how indeed could he not have thought what he said in his heart, since saying it in his heart is the same as thinking it? But if he really thought it because he said it in his heart, and did not say it in his heart because he could not possibly have thought it - and that seems to be precisely what happened - then there must be more than one way in which something can be said in one's heart or thought. For a thing is thought in one way when the words signifying it are thought, and it is thought in quite another way when the thing signified is understood. God can be thought not to exist in the first way but not in the second. For no one who understands what God is can think that he does not exist. Even though he may say those words in his heart he will give them some other meaning or no meaning at all. For God is that greater than which cannot be thought. Whoever understands this also understands that God exists in such a way that one cannot even think of him as not existing.
Thank you, my good God, thank you, because what I believed earlier through your gift I now understand through your illumination in such a way that I would be unable not to understand it even if I did not want to believe you existed.
Anselm now proceeds to deduce God's nature from the same basic definition of him as something greater than which cannot be thought.. He arrives as all the standard attributes: creative, rational, omnipotent, merciful, unchangeable, just, eternal, etc. It is, in effect, a theological tour de force.
Anselm's thoughts did not go unchallenged, however. His first major critic was Gaunilo, a monk in the abbey of Marmoutier. Gaunilo's reply is the only bit of writing we possess by him, which is a shame, because in it we encounter a very perceptive mind, although a radically different one than Anselm's. 

GAUNILO: HOW SOMEONE WRITING ON BEHALF OF THE FOOL MIGHT REPLY TO ALL THIS
To one who questions whether (or simply denies that) there exists something of such a nature that nothing greater can be imagined, it is said that its existence is proved in the first place by the fact that anyone denying it already has it in his thought, since upon hearing it said he understands what is said; and in the second place by the fact that what he understands necessarily exists not only in the mind but in reality as well. Thus its existence is proved, because it is a greater thing to exist in reality as well than to exist in the mind alone, and if it exists only in the mind, then what exists in reality as well will be greater, and thus that which is greater than all else will be less than something else and not greater than all else, which is nonsense. Thus what is greater than all else must necessarily exist, not only in the mind (which has already been acknowledge to be the case), in reality as well, or else it could not be greater than all else.
But perhaps the fool could reply that this thing is said to exist in my mind only in the sense that I understand what is said. For could I not say that all sorts of false and completely nonexistent things exist in my mind since when someone speaks of them I understand what is said? Unless perhaps what is being said here is that one entertains this particular thing in the mind in a completely different way than one thinks of false or doubtful things, and thus what is being said is that having heard this particular thing I do not merely think it but understand it, for I cannot think of this thing in any other way except by understanding it, and that means understanding with certainty that it actually exists. But if this is true, then in the first place there will be no difference between first entertaining that thing in the mind and then understanding that it exists. Imagine the case of that picture which is first in the painter's mind, then exists in reality. It seems unthinkable that, once such an object was spoken of the words heard, the object could not be thought not to exist in the same way God can be thought not to exist. For if God cannot be thought not to exist, then what is the point of launching this whole argument against someone who might deny that something of such a nature actually exists? And in the second place, this basic notion - that God is such that, as soon as he is thought of, he must be perceived by the mind as unquestionably existing - this notion, I say, must be proved to me by some unquestionable argument, but not by the one offered here, namely that this must be in my understanding because I understand what I'm hearing. For as far as I am concerned one might say the same thing about other things that are certain or even false, things about which I might be deceived (as I believe I often am).
Thus the example of the painter who already has in his mind the picture he is about to produce cannot be made to support this argument. For that picture, before it comes into being, exists in the art of the painter, and such a thing existing in the art of some painter is nothing other than a certain part of his understanding; for as Saint Augustine says, "If a craftsman is going to make a box, he first has it in his art. The box he actually produces is not life, but that in his art is life, because the artisan's soul, in which all such things exist before they are brought forth, is alive. And how are these things alive in the living soul of the artisan unless because are nothing other than the knowledge or understanding of the soul itself? But leaving aside those things which are known to belong to the nature of the mind itself, in the case of those things which are perceived as true by the mind through hearing or thought, in this case there is a difference between the thing itself and the mind which grasps it. Thus even if it should be true that there is something greater than which cannot be thought, this thing, whether heard or understood, would not be like the as-yet-unmade picture in the painter's mind.
Moreover, there is the point already suggested earlier, namely that when hear of something greater than all other things which can be thought of - and that something can be nothing other than God himself - I can no more entertain a thought of this being in terms of species or genera familiar to me than I can entertain such a thought of God himself, and for this reason I am able to think he does not exist. For I have not known the thing itself and I cannot form a similitude of it from other things. For if I hear about some man completely unknown to me, whom I do not even know exists, I could at least think about him through that specific and generic knowledge by which I know what a man is or what men are like Yet it could be true that, because the speaker was lying, the man I thought about actually did not exist at all, even though I had thought of him as an existing thing, my idea of him being based, not on knowledge of this particular man, but on knowledge of man in general. But when I hear someone say "God" or "something greater than everything else" I cannot think of it as I thought of that nonexistent man, for I was able to think of the latter in terms of some truly existing thing known to me, while in the former case I can think only of the bare words, and on this basis alone one can seldom or never gain any true knowledge. For when one thinks in this way, one thinks not so much of the word itself - which, insofar as it is the sound of letters or syllables is itself a real thing, but of what is signified by the sound heard. But a phrase like "that which is greater than everything else" is not thought of as one thinks about words when one knows what they mean. It is not thought of, that is, as one thinks about something he knows is true either in reality or in thought alone. It is thought of, instead, as one does when he does not really know what the words mean, but thinks of it only in terms of an affection produced by the words within his soul, yet tries to imagine what the words mean. On this basis, though, it would be amazing if he was ever able to penetrate to the truth of the thing. It is in this way and only in this way that this being is in my mind when I hear and understand someone saying there is something greater than everything else that can be thought of. So much for the claim that the supreme nature already exists in my mind.
Nevertheless, that this being must exist not only in my mind but in reality as well is proved to me by the following argument: If it did not, then whatever did exist in reality would be greater, and thus the thing which has already been proved to exist in my mind will not be greater than everything else. If it is said that this being, which cannot be conceived of in terms of any existing thing, exists in the mind, I do not deny that it exists in mine. But through this alone it can hardly be said to attain existence in reality. I will not concede that much to it unless convinced by some indubitable argument. For whoever says that it must exist because otherwise that which is greater than all other beings will not be greater than all other beings, that person isn't paying careful enough attention to what he says. For I do not yet grant, in fact I deny it or at least question it, that the thing existing in my mind is greater than any real thing. Nor do I concede that it exists in any way except this: the sort of existence (if you can call it such) a thing has when the mind attempts to form some image of a thing unknown to it on the basis of nothing more than some words the person has heard. How then is it demonstrated to me that the thing exists in reality merely because it is said to be greater than everything else? For I continue to deny and doubt that this is established, since I continue to question whether this greater thing is in my mind or thought even in the way that many doubtful or unreal things are. It would first have to be proved to me that this greater thing really exists somewhere. Only then will we be able to infer from the fact that is greater than everything else that it also subsists in itself.
For example, they say there is in the ocean somewhere an island which, due to the difficulty (or rather the impossibility) of finding what does not actually exist, is called "the lost island." And they say that this island has all manner of riches and delights, even more of them than the Isles of the Blest, and having no owner or inhabitant it is superior in the abundance of its riches to all other lands which are inhabited by men. If someone should tell me that such is the case, I will find it easy to understand what he says, since there is nothing difficult about it. But suppose he then adds, as if he were stating a logical consequence, "Well then, you can no longer doubt that this island more excellent than all other lands really exists somewhere, since you do not doubt that it is in your mind; and since it is more excellent to exist not only in the mind but in reality as well, this island must necessarily exist, because if it didn't, any other island really existing would be more excellent than it, and thus that island now thought of by you as more excellent will not be such." If, I say, someone tries to convince me though this argument that the island really exists and there should be no more doubt about it, I will either think he is joking or I will have a hard time deciding who is the bigger fool, me if I believe him or him if he thinks he has proved its existence without having first convinced me that this excellence is something undoubtedly existing in reality and not just something false or uncertain existing in my mind.
In the meantime, this is how the fool answers. If it is asserted in the first place that this being is so great that its nonbeing is logically inconceivable (this in turn being proved by nothing except that otherwise it would not be greater than all other beings), then the fool can answer, "When did I say that such a being, namely one greater than all others, actually exists, thus allowing you to proceed from there to argue that it so really exists that its very nonexistence is inconceivable?" It should first be proved conclusively that some being superior to (that is, greater and better than) all others exists, so that on this basis we can go on to prove the attributes such a greater and better being must possess. When, however, it is said that this highest being cannot be thought of as not existing, perhaps it would have been better to say that its nonbeing or the possibility of its nonbeing is unintelligible. For strictly speaking false things are unintelligible even though they can be thought of in the same way the fool thought God did not exist. I am absolutely certain that I exist, although I nevertheless know that my nonexistence is possible. And I understand without doubting it that the highest thing there is, namely God, exists and cannot not exist. I do not know, however, whether I can think of myself as nonexistant when I know for certain that I exist. If it turns out that I can do so in this case, why should I not be able to do the same concerning other things I know with equal certainty? If I cannot, though, the impossibility of doing so will not be something peculiar to thinking about God.
The other parts of that book are argued with such veracity, brilliance and splendor, and filled with such value, such an intimate fragrance of devout and holy feeling, that they should in no way be condemned because of those things which, at the beginning"it also prove that he exists are rightly intuited but less firmly argued. Rather those things should be argued more robustly and the entire work thus received with great respect and praise.

ANSELM'S REPLY TO GAUNILO
Since whoever wrote this reply to me is not the fool against whom I wrote in my treatise but instead one who, though speaking on behalf of the fool, is a catholic Christian and no fool himself, I can speak to him as a catholic Christian.
You say - whoever you are who claim that the fool can say these things - that something greater than which cannot be thought of is in the mind only as something that cannot be thought of in terms of some [existent thing known to us]. And you say that one can no more argue, "since a being greater than which cannot be thought of exists in my mind it must also exist in reality," than one can argue, "the lost island certainly exists in reality because when it is described in words the hearer has no doubt that it exists in his mind." I say in reply that if "a being greater than which cannot be thought of" is neither understood nor thought of, nor is it in our understanding or our thought, then God either is not that greater than which cannot be thought of or he is not understood or thought of, nor is he in the understanding or mind. In proving that this is false I appeal to your faith and conscience. Therefore "a being greater than which cannot be thought of" is really understood and thought of and it really is in our understanding and thought. And that is why the arguments by which you attempt to prove the contrary either are not true or what you think follows from them does not follow from them at all.
Moreover, you imagine that although "a being greater than which cannot be thought of" is understood, it does not follow that it exists in our understanding nor does it follow that, since it is in our understanding, it must exist in reality. I myself say with certainty that if such a being can even be thought of as existing, it must necessarily exist. For "a being greater than which cannot be thought of" cannot be thought of except as having no beginning; but whatever can be thought of as existing yet does not actually exist can be thought of as having a beginning. Therefore "a being greater than which cannot be thought of" cannot be thought of yet not actually exist. Therefore, if it can be thought of, it necessarily exists.
Furthermore, if it can be thought of at all, it must necessarily exist. For no one who denies or doubts the existence of "a being greater than which cannot be thought of" denies or doubts that, if it did exist, it would be impossible for it not to exist either in reality or in the mind. Otherwise it would not be "a being greater than which cannot be thought of." But whatever can be thought of yet does not actually exist, could, if it did come to exist, not existence again in reality and in the mind. That is why, if it can even be thought of, "a being greater than which cannot be thought of" cannot be nonexistent.
But let us suppose that it does not exist (if it is even possible to suppose as much). Whatever can be thought of yet does not exist, even if it should come into existence, would not be "a being greater than which cannot be thought of." Thus "a being greater than which cannot be thought of" would not be "a being greater than which cannot be thought of," which is absurd. Thus if "a being greater than which cannot be thought of" can even be thought of, it is false to say that it does not exist; and it is even more false if such can be understood and exist in the understanding.
I will go even farther. Without doubt whatever does not exist somewhere or at some time, even if it does exist somewhere or at some time, can be thought of as capable of as existing never and nowhere, just as it does not exist somewhere or at some time. For what did not exist yesterday and exists today can be thought of as never existing, just as it is thought of as not having existed yesterday. And what does not exist here but does exist somewhere else can be thought of as not existing anywhere. And it is the same with something some parts of which are absent at times. If that is the case, then all of its parts and thus the thing in its entirety can be thought of as existing never and nowhere. For if it is said that time always exists and the world is everywhere, it is nevertheless true that time as a whole does not exist forever, nor does the entire world exist everywhere. And if individual parts of time exist when other parts do not, they can be thought of as never existing at all. And just as particular parts of the world do not exist where other parts do, so they can be thought of as never existing at all, anywhere. And what is composed of parts can be broken up in the mind and be nonexistent. Thus whatever does not exist as a whole sometime or somewhere can be thought of as not existing, even if it actually exists at the moment. But "a being greater than which cannot be thought of," if it exists, cannot be thought of as not existing. Otherwise it is not "a being greater than which cannot be thought of," which is absurd. Thus it cannot fail to exist in its totality always and everywhere.
Do you not believe that the being of which these things are understood can be thought about or understood or be in the thought or understanding to some extent? For if he is not, then we cannot understand these things about him. If you say that he is not understood or in the understanding because he is not fully understood, say as well that one who cannot look directly at the sun does not see the light of day, which is nothing other than the light of the sun. Certainly "a being greater than which cannot be thought of" is understood and exists in the understanding at least to the extent that these statements about it are understood.
Anselm continues as some length, but much of what he says seems repetitive. He does eventually note one important difference in the way he and Gaunilo have been phrasing the matter..
You often picture me as offering this argument: Because what is greater than all other things exists in the understanding, it must also exist in reality or else the being which is greater than all others would not be such. Never in my entire treatise do I say this. For there is a big difference between saying "greater than all other things" and "a being greater than which cannot be thought of." If someone says "a being greater than which cannot be thought of" is not something actually existing or is something which could possibly not exist or something which cannot even be understood, such assertions are easily refuted. For what does not exist is capable of not existing, and what is capable of not existing can be thought of as not existing. But whatever can be thought of as not existing, if it does actually exist, is not "a being greater than which cannot be thought of."
Anselm goes on to present his standard argument that the nonexistence of such a being is inconceivable. Then he adds a key observation.
It is not, it seems, so easy to prove the same thing of "that which is greater than all other things," for it is not all that obvious that something which can be thought of as not existing is not nevertheless greater than all things which actually exist.