The best authentication has three components:
This is an old mantra that I don't think is believed anymore (except by companies that sell biometric systems of course.
Numbers 2 and 3 are essentially the same...they are both something you have. The idea that number 3 is somehow different from number 2 stems from the assumption that biometrics does something special, like it's uncopyable. It's not magical though and it really is just something you have.