I'm somewhat of a time-travel expert. Thusly, I will now explain something no one ever seems to understand.
The so-called "Grandfather Pair o' Docs":
My grandfather gave me a sweet pair of Docs. If I were to put on Grampa's Docs and then go back in time and steal Grampa's Docs before he could give them to me, he could never give them to me; clearly then I could not be wearing them. But I am.
Plus, once I steal Grampa's Docs, there are now entirely too many Docs. This cannot happen.