Probable vs. Possible

Because it's my blog and I can be pedantic if I want to, I'd like to submit a short screed on one of my pet peeves in the common parlance: the erroneously either lazy or synonymous use of "probable" and "possible."

"Probable" means "likely to be the case or to happen." The likelihood is of a statistical or empirical sort: given what we know about things or events like this, it is probable that X is going to be the case/happen.

"Possible" means "able to be or to be done." At its base, this is pure possibility: if the thing in question does not entail a logical contradiction, then this thing can be or be done.

"Probable" involves likelihood, "possible" involves feasibility.

Yet we'll hear statements like the following:
"Given that John is late for every meeting, it's possible he'll be late again today."
"With so many billions of galaxies and billions of stars in each galaxy, it's probable there's intelligent life on other planets."

I think we need a switch here!

The first sentence provides a basis for predicting John's behavior then proceeds to make a prediction. The speaker is making the claim that, given his past behavior, it is likely John will be late for the meeting. Likely = probable, so it would be better to say "it's probable he'll be late again today."

The second sentence would appear to be like the first, setting out criteria by which to evaluate the likelihood of something, but the error here is less formal than material: the probability of intelligent life existing elsewhere in the universe can only be based on either 1) an extrapolation from the known quantity (which, in this case, is just one), or 2) an extrapolation from knowledge of all of the requirements for life (which is awfully tricky, since we don't know all the variables involved, and besides we know only one form of life--carbon-based organic life; there could be others). By the first criterion, the probability is "one in the whole universe," and by the second, it's really unknown. The speaker in this sentence would be better served saying that it's "possible there's intelligent life on other planets."

Remember: probable = likely, possible = feasible. I think it's possible you'll never make this mistake again, but it's probable you will.