Hi guys, how's it going? I'm trying to identify the source of a commonly cited quote by Lytton Strachey that goes as follows: "Perhaps the best test of a man's intelligence is his capacity for making a summary."

Searching for this serves me countless results from quotes-websites, with no reference to where it was written / said and when. Do any of you know this? I'm beginning to wonder if it really is a genuine quote.