Progress on this path ought not to be imagined as moving in a direct, uninterrupted line. In practice it follows a wavelike course. The mind rises vigourously to the crest of its powers for a time and then, tired, sinks into the trough. Here it remains for a while resting and then begins the same alternation.

-- Notebooks Category 2: Overview of Practicies Involved > Chapter 3: Uncertainties of Progress > # 50