why do we use "the" in one of this sentence but not the other?

[Ketap]

hi..i am new here..and i am glad to see this forum..btw, i want to ask..

why do we use "the" before this sentence : exercise is good for the body
i read once a book which said that the reason is because it is placed before a noun which describe a general type of thing..

but on the other hand...we also have a rule for not using "the" if we are talking general things,such as "dogs hate cats",etc..

i have never been able to differentiate between those two..i guess i am confused...please help.thx
regards,Ketap

[Alan]

You could regard 'the body' here as a kind of fixed expression (more natural than 'bodies').