What’s the difference between probable and possible? Strictly speaking, they’re unrelated, but in popular usage, their distinction is merely one of degree.
Possible — the noun form is possibility — means “having the potential.” (Potential, along with the latter word’s root, potent, shares an etymological origin with the former word.) Possible stems from the Latin term possibilis, which derives in turn from posse, which means “power” or “to be able.” Posse itself was borrowed into English from the Medieval Latin phrase posse comitatus, which literally means “power of the county.”
(This term, later shortened to posse, referred to the authority of a local official to conscript men to respond to an emergency; such a deputized detail features in many works of filmed or printed fiction in the western genre, but now, the term is most commonly heard as a jocular slang synonym for a celebrity’s entourage or retinue or anyone’s group of friends.)
Probable, which means “likely,” comes from the Latin term probabilis, which itself stems from probare, meaning “to approve, prove, or test.” Related words referring to the first sense include approbation and probity; prove itself is akin to probable, as is probe. Probability is the term for the branch of mathematics dealing with chance and is used in logic to refer to the degree to which two statements confirm each other.
Probable refers to what is likely to be done, to occur, or to be true; possible refers to what can be done, to occur, or to be true. If you say something is probable, you are expressing more confidence about it than if you state that it is possible. But the distinction is significant: It is possible, for example, for anyone to become fabulously wealthy, but the probability is infinitely variable.