I was in the debate team in high school. And there were times that our team would take the against side of the statement. In his famous work Prologion, written in 1077-1078, Anselm presents the idea the God exists because God is the greatest thing of all, that the idea of thinking of God exists prove its existence. Hundred of years later, Thomas Aquinas brings up the account that addresses Anselm's idea in objection 2 of Question II, First Article of Summa Theologica. Aquinas objects Anselm's argument later in his work by attacking the idea that God is something that can be thought greater.
To understand Anselm's argument for God's existence, one must first understand the principles that forms the argument. The first principle is the claim that “nothing greater can be thought.” There is too types of existing, existing in understanding (existing0) and existing in reality (existing1). Then, we try to think of something is existing1. Anselm let “something” be “something than which nothing greater can be thought (NGT),” or in another word “a being than which nothing greater can be conceived” according to the Proslogion. The very first act of thinking that something is existing1 serves as the basis of it existing0. Because in the process of trying to think of something greater, we already establish its existence0. For example, there is a flower A, existing1. This flower A is the most beautiful flower, that this flower is a NGT. In order to prove this flower is NGT, one has to think of all the flower he has seen, flower B, C, or D. In the process of searching through one's mind trying to think of a flower that is more beautiful than flower A, flower A already exists in one's mind, which is existing0. To open his argument, Anselm then said an example of NGT is God.
The second principle is the principle of “thinking of non-existing objects (Principle E).” Anselm uses the example of a painter conceiving the drawing he will paint, then executing his plan in mind to make the painting exists in reality. To breakdown his argument, let's begin with something that exists0. Because one has the ability to imagine that something exists0, one can also imagine that the same thing exists1. This priniciple is the most uncontroversial because it is just a matter of imagination. The fact that one has imagination allows him to imagine whatever he wants. For example, one imagines he won the lottery and has a million dollars. It is safe to say that one can proceed to think of that million dollars is existing1, regardless of if the million dollars truly exists1. Applying Principle E back to the argument of NGT, if one can imagine NGT in existence0, one can also go ahead and imagine NGT existing1.
The third principle is “thinking something greater than something (Principle G). This principle compares the greatness of existing0 and existing1. Anselm proposes that “greatness” has two faces: qualitatively and existentially. To further support his argument, Anselm implies that greatness qualitatively means everything positive, such as, more beautiful, more knowledgeable, more influential, etc. Base on this assumption, Anselm argues that, “Suppose it exists in the understanding alone: then it can be conceived to exist in reality; which is greater.” To apply this principle in daily life, one can relate to a million dollars example above. One can imagine having a million dollars in his mind, but that million dollars doesn't exist1. In another case, one owns a millions dollars, which means that million dollars exists1. The rhetorical question here will be, which case is more appealing to you, which scenario is “greater?” Back to Anselm's claim, the “it” in his argument is, of course, NGT, which is God. That God exists1 is better than God only exists0 but not exists1.
The next principle returns to the basic meaning of NGT. Principle N presents the idea “if something can be thought to be...