"Well it certainly wasn't my own choice," Strange explains, as if it's obvious. "I didn't even know it was possible to do magic until the man under the hedge sold me a spell, rambled a story or a prophecy of some sort, and told me I was a magician." The whole situation was odd. A little part of Strange actually wishes he had paid attention to Vinculus's prophecy, but a larger part of him is glad he didn't. Why would he want to govern his life by the words of a madman that may or may not have been true in the first place?

But if it was chosen for him...well, who honestly would choose Strange to do magic to begin with? There were plenty of theoretical magicians back in York, before Norrell dissolved their society. If it was some higher power that gave him the magic, surely it would be more logical to choose someone who's studied magic instead of someone who never gave it a moment's notice until recently. Not that he was complaining, of course. He was simply judging Lambert's hypothetical question.

"I think that the best way of describing how I came across my magic is that I stumbled into it."