SimchaMinute by Rabbi Avigdor Miller
For the people who think about the chovos halevavos (the duties of the mind), their mind is like a chamber which has become illuminated.
It’s like when the light goes on in a dark room and the whole room is filled with light. So it is that an individual’s character becomes changed because of the great influence these thoughts have on him. That’s why [wisdom] is the light of the mind." — Ohr Avigdor Hakdama