Even in the Psalm from which these words are taken the promise to the meek is not held forth as an arbitrary reward, but as having a kind of natural fulfilment. For they shall inherit the earth. The wolves devour the sheep, yet there are more sheep in the world than there are wolves, and the sheep, continue to multiply, and to feed in green pastures. Those who hunger and thirst. Hunger and thirst express a deep longing.
Righteousness is the divine order, divine law.
This beatitude is concerned with personally sharing in this righteousness, hence with personal righteousness, responding to the will of God. Here Jesus turns one of the elemental human instincts to spiritual use. There is in all men hunger for food, for love, for God. It is passionate hunger and thirst for goodness, for holiness.