Question

I saw that many people had already asked about this, but I didn’t understand, so I’ll ask too.
Rabbi writes in the notebook:
If the concept of God existed only in the mind and not in reality, then one could conceive of something (a concept) greater than it: God who exists (understanding + translation neurons)[1]. It is important to note that the comparison here is between two images[2] in consciousness (the cognitive image of X and the cognitive image of X as existing), and not between X itself and the concept of X, as we clarified above. Anselm argues that there is nothing preventing us from also conceiving the image of God as existing, and this image is greater than the image of the abstract concept of God (without existence). But if so, then in our consciousness there is another concept besides God that is greater than Him, and yet we can still conceive it.
But that of course contradicts the definition of the concept God as the greatest being that can be conceived (or that no greater can be conceived. Here, we have conceived something else greater than Him). Therefore the fool’s hypothesis that God does not exist leads us to a contradiction. The concept God includes its existence within it, and so God who does not exist is like a round triangle (or a stone that the omnipotent cannot lift). This is a proof by negation that the fool’s view that God does not exist is incorrect, and necessarily he and we must adopt the opposite hypothesis, that God exists (that is, is realized in reality).
 
And I tried to understand, but couldn’t, so if the Rabbi can help me I’d appreciate it.
Anselm assumes that if something can be conceived in the mind and does not also exist in reality, then it can also be conceived as existing. In the note the Rabbi says that clearly this is not referring to a perception of existence that comes from visual translation neurons, because even Anselm agrees that one does not see God. My question is whether there is an assumption here in the argument. I can certainly think of an entity than which no greater can be conceived, but I am only thinking about it, and I can’t arrive at a perception of an ‘existing concept’ like the one Anselm is talking about. Is that really an assumption? Or did I miss something? Thanks