(to make) a conclusion, opinion, based on incomplete evidence

speculation, assumption

to form an opinion, give an answer, make a statement about (something) that is not based on very careful thought, calculation or definite knowledge

belief, speculation

a vague idea

idea, clue

something which seems likely although there is no proof; arrogance (the usual word)

belief, hypothesis

(an instance of) speculating; (a business deal as a result of) speculating

theory, guess

an act of suggesting

advice, plan

the act of supposing something to be true but without proof; a guess

guess, belief

to guess (the usual word)

guess, conclusion