Thus have I heard - At one time, the Blessed One was dwelling in Sāvatthi, in Jeta's Grove, Anāthapiṇḍika's park. Now, at that time, the Blessed One was sitting, reviewing the multitude of harmful and unwholesome mental qualities he had abandoned, and the multitude of wholesome mental qualities that he had developed to completion.

Then the Blessed One, having perceived that multitude of harmful and unwholesome qualities had been abandoned by him, and that multitude of wholesome qualities had been developed to completion, at that time uttered this inspired utterance:

"There was in the past, then there was not, there was not in the past, then there was; It neither was, nor will it be, nor does it exist now." [1]

[1] I suspect that the verse refers to a review of harmful and unwholesome mental qualities done one at a time. This process is to show that an awakened being carefully examines and verifies things as they have come to be instead of assuming or jumping to conclusions.