That which is like poison at first but like nectar in the end — that happiness is called sattvic, born of the clarity of a mind turned toward the Self.
Explanation
Krishna is describing the three kinds of happiness (sukha), and begins with the sattvic kind, shaped by clarity (sattva), the clear and balanced guṇa. It is hard and bitter at the start, because it takes discipline, but it ripens into lasting joy. Its source is the serenity (prasāda) of the intellect (buddhi) in relation to the self (ātman); some readers take ātma-buddhi as one's own mind in its clarity, others as the understanding that is directed to the Self.