Multi-Factor Authentication Concern

[jsewell () jsewell com]

I'm having an argument with someone at work about multi-factor authentication. We'll call him Bob.

Bob claims that in a multi-factor authentication system, the factors don't need to identify the same person. In other 
words, Bob thinks it's perfectly OK for the door to the data-center to open when Jim badges in, Mike scans his retina, 
and Sally enters a her PIN.

This is obviously wrong. Bob says "prove it". So I've scoured the net and books for something that describes 
multi-factor authentication as requiring that all factors identify the same person. So far, I can't find anything.

Is it so obvious that nobody has bothered to write it down, or am I wrong in my thinking?

Thanks!