(Latin for I'm speaking from experience).
This was a popular phrase one of my professors at Gregorian University used a lot. I suppose he meant it as the final proof that what he was saying was right.
Ironically, now, some twenty years later, all I remember from his lectures was that phrase...
That is not to say he did not teach me anything. He was an excellent teacher, and a brilliant canonist. What I mean by "all I remember" is his exact words. I still remember many a subtle advice he gave us, just not necessarily the exact words he used.
For example, one of his tidbits was the advice never to take on a case of a bishop who asks for a dispensation to get married. He told us that never in Church history was such a dispensation granted, and that a smart canonist never takes on a case he cannot win.
He had to teach me well: In all the years I was a Catholic priest and practiced Canon Law, I never lost a case.