The true way (of learning) so that it remains is to understand, it is not to learn by heart. You learn something by heart, it is mechanical, you see; but after some time it will be effaced, unless you make use of it constantly. For example, you are made to learn by heart the multiplication tables; if you constantly use them, you will remember them, but if by chance for years you remain without using them, you will forget them completely. But if you understand the principle, you will be able to remember them.
…If you understand the thing, if you have the sense of the principle which is behind, you can remember it indefinitely, for hundreds of years if you live for hundreds of years; whereas something you have learnt by heart… after some time the braincells multiply, are replaced, and some things are wiped out.
Ref: Questions and Answers 1955