Possibility is one of my favorite topics. Here is a collection of quotes I have grown to appreciate over the years.
Aristotle: "A sea-fight must either take place tomorrow or not, but it is not necessary that it should take place to-morrow, neither is it necessary that it should not take place, yet it is necessary that it either should or should not take place tomorrow."
Aristotle: "One of the two propositions in such instances must be true and the other false, but we cannot say determinately that this or that is false, but must leave the alternative undecided. One may indeed be more likely to be true than the other, but it cannot be either actually true or actually false. It is therefore plain that it is not necessary that of an affirmation and a denial, one should be true and the other false. For in the case of that which exists potentially, but not actually, the rule which applies to that which exists actually does not hold good."
Robert Musil: "To pass freely through open doors, it is necessary to respect the fact that they have solid frames. This principle, by which the old professor had lived, is simply a requisite of the sense of reality. But if there is a sense of reality, and no one will doubt that it has its justifications for existing, then there must also be something we can call a sense of possibility. Whoever has it does not say, for instance: Here this or that has happened, will happen, must happen; but he invents: Here this or that might, could, or ought to happen. If he is told that something is the way it is, he will think: Well, it could probably just as well be otherwise. So the sense of possibility could be defined outright as the ability to conceive of everything there might be just as well, and to attach no more importance to what is than to what is not. The consequences of so creative a disposition can be remarkable, and may, regrettably, often make what people admire seem wrong, and what is taboo permissible, or, also, make both a matter of indifference. Such possibilists are said to inhabit a more delicate medium, a hazy medium of mist, fantasy, daydreams, and the subjunctive mood. Children who show this tendency are dealt with firmly and warned that such persons are cranks, dreamers, weaklings, know-it-alls, or troublemakers. Such fools are also called idealists by those who wish to praise them. But all this clearly applies only to their weak subspecies, those who cannot comprehend reality or who, in their melancholic condition, avoid it."
Søren Kierkegaard: "It is spirit to ask about two things. (1) Is what is being said possible? (2) Am I able to do it? It is to lack spirit to ask about two things: (1) Did it actually happen? (2) Has my neighbor done it; has he actually done it? In asking with regard to my own actuality, I am asking about its possibility, except that this possibility is not esthetically and intellectually disinterested but is a thought-actuality that is related to my own personal actuality-namely that I am able to carry it out. The how of the truth is precisely the truth."
Mahatma Gandhi: "Man often becomes what he believes himself to be. If I keep on saying to myself that I cannot do a certain thing, it is possible that I may end by really becoming incapable of doing it. On the contrary, if I have the belief that I can do it, I shall surely acquire the capacity to do it even if I may not have it at the beginning."
Giorgio Agamben: "Remembrance restores possibility to the past, making what happened incomplete and completing what never was. Remembrance is neither what happened nor what did not happen but, rather, their potentialization, their becoming possible once again."