The Numerical Discourses (Anguttara Nikaya, also called the Gradual Sayings) begin with "The Mind." The Buddha -- who knew many things, perhaps all that it is possible to know -- knew of nothing more beneficial than the well developed mind of a meditator:
No other thing do I know, O meditators, that brings so much harm as a mind that is untamed, unguarded, unprotected, and uncontrolled. Such a mind indeed brings much harm.
- More definitions: BUDDHIST DICTIONARY (Nyanaponika)