classification – Why do you need at least ln (n!) many comparisons to order a list?

"If each item comparison (test if $ a_i le a_j $ ) provides at most a bit of information, argues that it needs at least in the order of $ ln (n!) $ Many tests / comparisons to sort the list. "

This is a question about my homework at this time. I do not understand how I'm supposed to discuss this, since I do not understand why it's true. Any pointer in the right direction? Thank you.