How do we think?

When I say "apple" ,an apple will appear in your mind. You can understand it. People may explain it as some correspondence between images and specific signs. And the signs themselves are images so actually we associate the images with sounds and then sounds with signs. Humans have evolved so much that today they can understand written language in silence, not needing to read them out.

The understand of concrete things may be as easy as the above. But what about that of abstract things, concepts and statements? That should be hard to explain because explanation itself is an abstract statement, which I must use my understanding system of abstract things to understand.

I used to guess, the approach to solve this problem is up to science. That is, to study the brain. But there is still a critical obstacle that we still have to think using our brain.

Is that the unreachable acme of human cognition? Do humans have to continue evolving to answer it? Or never?