Answer:
Define (say).
Step-by-step explanation:
A dream can be defined as series of imaginary (unreal) events that a person sees in his mind while sleeping. Thus, in order to have a dream or dream about anything at all, the person must be asleep i.e a semi-unconcious state.
Dreams have fascinated philosophers for thousands of years, but none could define (say) specifically what dream actually is.
The most appropriate word to fill up the blank is "define" or "say" because none of the philosophers can state specifically what dream actually is.