If something is reducible to X, you can treat the whole complicated thing as really just X — you can boil it down and count it as that. Philosophers and scientists use it a lot: ‘the mind is reducible to the brain’ means they claim mind-talk is nothing more than brain-talk. It is a formal, academic adjective, not kitchen-table English. There is a matching opposite, irreducible, for something you cannot boil down any further. People also use reducible in maths for a fraction you can simplify; that is the same ‘make simpler’ idea, but the sense you need here is ‘able to be considered simply as’. When someone says a rich thing is reducible to one cause, they may be explaining clearly — or, though actually, throwing too much away.

可化約的