James Anderson's Test career is five and a half years old and he still has not worked out why he can bowl like a demon one day and a dunce the next.
His experiences this year have illustrated the point perfectly: brilliant in the second Test on tour against New Zealand in Wellington in March, dreadful a week later in Napier; first class in the opening home Test at Lord's, second rate at Old Trafford, and back to his destructive best again here.
He must be a nightmare for his captain, not to mention the selectors, although one suspects no one finds Anderson more infuriating than he does himself. England employ a sports psychologist as well as a clutch of technical coaches. Anderson must be one of his more challenging clients.
What the Lancashire bowler does know is that whether or not he performs depends largely on confidence, although it probably did not need a highly qualified mental guru to work that out. Yesterday it seemed to be there in abundance.
Perhaps he remembered taking five wickets against South Africa here in August 2003, his second such haul in a debut summer that propelled him at breathtaking speed to the front rank of England's bowling assets at only 21 and in only his 22nd first-class match.
Or maybe it was something as simple as the tingle he must have felt yesterday morning as he left the field to an ovation granted in circumstances he had not experienced before in a Test, for his batting. Having emerged from his appearance at No 9 on Thursday to be a competent ally for Stuart Broad in a 76-run partnership for the eighth wicket, he had handsomely surpassed his best score in Tests.
Little things like that can make the difference to a sportsman with a psyche as unreliable as his. How he must wish for something of what Broad possesses. Like Anderson, the Nottinghamshire tyro has been unleashed at this level with his L-plates barely discarded. So far, though, this 21-year-old has total self-belief.
Unfazed by the troubles encountered by others with more experience on a less than straightforward pitch, Broad set himself determinedly on his goal of achieving a maiden Test fifty on his home ground and got there, despite being twice dropped.
It was a lovely moment, and one that would have been sweeter still had his father, Chris, who graced this ground so often, managed to time his business commitments better. The former England opener was here on Thursday and will be again today, but had to be elsewhere yesterday.