K, as I understand it, is saying that thought is limited because thought is based on knowledge, and knowledge is limited.
The evidence for the fact that knowledge is limited is that in the scientific field - where knowledge is essential - it is always being added to. There is no complete knowledge of the laws of nature, of the nature of the physical universe.
Even though in the discussion they agree to limit their inquiry about knowledge to psychological knowledge, my understanding is that this limitation applies to all knowledge, not just psychological knowledge.
Knowledge is limited for Krishnamurti because it is based on experience which is limited. So there is a chain of causation here: thought is limited because it is based on knowledge which is limited, and knowledge is limited because it is based on experience which is limited. What Krishnamurti means by the word experience is a little ambiguous, but I understand him to mean that ordinarily all experience is filtered through our conditioning.
There is, potentially, an unlimited manner or mode of experiencing - which may take place in what Krishnamurti calls true meditation, or when the mind is in a state of total attention. But this is not what Krishnamurti means by experience. He says that in true meditation there is no experience.
So experience - experience of traffic, experience of other people, experience of eating different foods - is registered in the brain by our thinking, and becomes our knowledge. So an experience is a combination of the background of our thinking and conditioning, as well as our sense perceptions of the world. The synthesis of our conditioning with what we sense-perceive becomes our experience. E.g. I ate an apple yesterday and it tasted a little sweet and a little sour at the same time. This experience of eating the apple was registered by the brain, and has become my knowledge. My memory, which is my knowledge, is an abstraction of the experience I had when eating the apple.
So, you see, the limitation of experience is one thing. And the storing up in memory of that experience is a further limitation because I must abstract from that experience to create a memory of it, which is my knowledge. Then thought: all thought, all thinking, is a projection of this background storehouse of my memory, which is my knowledge. And so thought is never new, it is always from the past - and my past knowledge is always limited (because it is itself an abstraction of what I experienced in the past). So thought is always limited.
Do you see the causal chain of it? This is what I understand Krishnamurti to be saying, although they only touch on aspects of this during their discussion.
I’m not sure I completely understand your question here?
They are saying - as far as I understand it - that the world, the universe, as well as the mind, is potentially limitless, and so there will always be some aspect of reality (or the mind) that thought cannot capture because thought is always limited. So thought is the known (it is a projection of our memory and knowledge), and reality in its wholeness is essentially unknown (and unknowable through thought, because thought is always limited).
This is what I understand them to be saying essentially (though they put it in slightly different words).