Chiara Marletto in Nautilus:
If you could soar high in the sky, as red kites often do in search of prey, and look down at the domain of all things known and yet to be known, you would see something very curious: a vast class of things that science has so far almost entirely neglected. These things are central to our understanding of physical reality, both at the everyday level and at the level of the most fundamental phenomena in physics—yet they have traditionally been regarded as impossible to incorporate into fundamental scientific explanations. They are facts not about what is—“the actual”—but about what could or could not be. In order to distinguish them from the actual, they are called counterfactuals.
Suppose that some future space mission visited a remote planet in another solar system, and that they left a stainless-steel box there, containing among other things the critical edition of, say, William Blake’s poems. That the poetry book is subsequently sitting somewhere on that planet is a factual property of it. That the words in it could be read is a counterfactual property, which is true regardless of whether those words will ever be read by anyone. The box may be never found; and yet that those words could be read would still be true—and laden with significance. It would signify, for instance, that a civilization visited the planet, and much about its degree of sophistication.