I think I shall not go too far in saying that a large number of the more scientific psychologists of to-day will at least agree that the brain as you find it in the adult man is very largely the result of the exercise of thinking through the earlier years of life.
I do not think they would go so far as to say that thinking has literally produced it.

They would, however, judging by very many things that have been said, be willing to admit that by hard thinking we can improve our apparatus of thought.

That is one reason for thinking hard--in order to think better.

And the harder you think, the more will your thinking instrument improve.
In my next step, however, I cannot by any stretching of ordinary science persuade it to accompany me, or give me a foundation; for the point is that your consciousness, working on the next plane above the one on which the organ of consciousness is being built, is the shaper of that mechanism.

To put it concretely: your physical brain is built up from the astral plane, and it is your consciousness working in matter finer than the physical which builds up the brain in the forming child within the limits laid down by karma.