Assume I know everything

Spotted on a T-shirt:

To save time, let’s assume I know everything.

I laugh, but this is actually how some people act, of course. There’s an “expert” I encounter regularly on Facebook who is the embodiment of this, er, slogan. He wants to tell you how it is, and he wants you to take his word for it — he invariably refuses to elaborate or substantiate, because he’s simply too busy. Not too busy to have posted his opinion in the first place, mind you! But too busy to back it up.