Do some philosophers regard thinking as a symbolic process only because they don't actually think for themselves -- rather, like most of us, they are "having thoughts", their subconsciousness (which itself is, technically, a neural network AI, a chatbot) literally speaking "their" thoughts to them?
Is it possible to argue that the basis of thinking is never symbolic -- it is always visual? We ourselves — as in "our rational mind", our rational Self — understand something by visualizing it, by creating a three-dimensional c̸o̸m̸p̸u̸t̸e̸r mental model of it, so, in effect, we could run a simulation of that portion of reality in our head. <== And that is how we know things, by acquiring a mental model for it.
"To know your Self, think for yourself."
Only thereafter we turn to symbolic language in order to to describe what we saw to others, to share our understanding (just as we use symbolic language to describe -- and, yes, share! -- our actual experiences). The reverse must be true as well -- we can really understand what we have been described with words when we visualize and re-enact that story in our imagination, re-living, to some extent, the storyteller's own experience.
Just to clarify -- as I understand it, this question is about understanding/knowing something, which is not the same as having an idea of it. Some things can never be known -- for example, no one can know what is a chair — one can only have an idea of what a chair is. "Simple ideas" like that (using John Locke's term) are always a product of the neural net in our sub-consciousness. And while the latter can have a very good idea of many things, ultimately it knows nothing and understands nothing -- because, relying on experiences alone, it has no concept of the "outside". And with no concept of the objective reality, there can be no concept of truth either!
In a way, our subconsciousness never knows, it merely pretends that it does -- "... for there is no truth in it. When it lies, it speaks its native language." (John 8:44)
That's why ultimately -- and it was true for Jesus, as it was for Socrates -- "I cannot teach anybody anything. I can only [try and] make them think."