“X is possible” means: in the present context of knowledge, there is some, but not much, evidence in favor of X and nothing known that contradicts X.
When you say “maybe,” you are saying there is at least some evidence, some reason to suspect X. This is a claim that must be justified. There are many fantasies that are outrightly impossible, because they contradict already known facts. And there are other fantasies that are mere arbitrary inventions; even if you cannot specify facts which contradict these inventions, you have absolutely no basis to hypothesize them.
It is possible, the skeptic argument declares, for man to be in error; therefore, it is possible that every individual is in error on every question. This argument is a non sequitur; it is an equivocation on the term “possible.”
What is possible to a species under some circumstances, is not necessarily possible to every individual member of that species under every set of circumstances. Thus, it is possible for a human being to run the mile in less than four minutes; and it is possible for a human being to be pregnant. I cannot, however, go over to a crippled gentleman in his wheelchair and say: “Perhaps you’ll give birth to a son next week, after you’ve run the mile to the hospital in 3.9 minutes—after all, you’re human, and it is possible for human beings to do these things.”
The same principle applies to the possibility of error.