Ah, I don’t recall… It was a long time ago.
Robert Spencer wrote a praised book on it, though: Did Muhammad Exist?: An Inquiry Into Islam’s Obscure Origins, 2014. Maybe he has answers.
What do you mean?! Next, you’ll wonder about the Ba’al, I meant Ba’al Shem Tov (sorry), Hershel, the Yoshke figure, the Khazar king of the Kuzari, and other famous Jews (?) (just having fun!)…
I didn’t read the book, but when academics speak of “existence”, they mean to say: Are all accounts referring to one and the same human being, or to several specimens?