"If you believe that momentum is conserved, define force as dp/dt and --
shazzam! -- you've got a nice, general, robust version of the third law."
Really? Didn't we just go through a lot of discussion about distinguishing
between a particular force and a sum of forces? I can't just replace F/-F
with dp/dt's, because dp/dt is equal to the NET force on an object. It
isn't equal to the F's in the historical Newton's 3rd law except in the
special case of there being just two objects that are interacting.