What do you mean, can you prove it???
Scenario #1 (simple 1d auth)
Server asks: Who you are?
I respond: I'm Bruce Weirdan.
Server asks: Can you prove that?
I respond: Yeah, of course. I know the password only Bruce knows (here it is: '********'), thus, rest assured, me is him.
*Server checks the data*
Server respond: Welcome, Bruce!
Scenario #2 (classic 3d auth)
Server asks: Who you are?
I respond: I'm Bruce Weirdan.
Server asks: Can you prove that?
I respond: Yeah, of course. I know the password only Bruce knows (here it is: '*********'), I have the hardware token issued to him (*inserting the token into the reader*), moreover, here is my retinal scan (*opening the eye for scanner to scan*) you could check against your database, thus, rest assured, me is him.
*Server checks the data*
Server respond: Welcome, Bruce!
Scenario #3 (delegated auth)
Server asks: Who you are?
I respond: I'm Bruce Weirdan.
Server asks: Can you prove that?
I respond: Yeah, of course. There's an authority you trust, they know me and could confirm my identity, thus, rest assured, me is him.
*Server asks the authority*
Server respond: Welcome, Bruce!
... and so on.