I think that profound knowledge is not for high heads." "I wonder just what you mean." "Oh, the moment you lose humility, you stop learning," the Doctor went on.

"There are two doors to every intellect.

One lets knowledge in, the other lets it out.

We must keep both doors in use.

The mind is like a purse: if you keep paying out money, you must, now and then, put some into your purse or it will be empty.