Why is the definite article used in expressions like the more and the less? For example,
The more you study, the more you know.
The less you study, the less you know.
It's not really the definite article, it just looks like the definite article. It's derived from the instrumental case form of the pronoun that in Anglo-Saxon (meaning "by that amount"), which has come to be pronounced exactly the same as the definite article over the centuries.