In the philosophy of language, talk of how things might be, or how things ought to be, is called modality. Imagine all the times we qualify a statement, by saying things like, “I hope that X,” “I believe that X,” “It is acceptable to do X,” all of those modifying clauses are called modal operators.
The most famous kind of modality is called alethic modality, which takes this form:
–It is possible that X
–It is contingently true that X
–It is necessary that X
–It is impossible that X
Here comes a challenge: when we make a modal statement, we seem to be referring to a fact about the universe, but it is not obvious where we find the fact of an unactualized possibility. Remember in high school the one stoner friend you had that said, “Oh, I could get straight-As if I tried?” Here, she is referring to a possible state of affairs, and is making a statement of contingent truth: If she tried hard, then it would be the case that she would get a straight-A report card. Most people would accept that this is a true statement, but its different than our intuitions about most statements of fact, because there is no available state of affairs to compare the statement to. If our friend said, “My backpack is pink,” the truth of that statement is generally understood to be relative to if the backpack IS ACTUALLY PINK. So how do we know it is true that if she tried hard, she actually would get all As? I tried hard in high school, and I didn’t get all As, this doesn’t seem like a given, but it might be true, regardless.
Or consider this, suppose I got into an argument with somebody about climate change. “Climate change is a serious issue that could threaten the survival of the human species,” I say. My opponent looks directly at me and says, “No, that’s impossible. God will not let the end of the world happen before the Second Coming. We have nothing to worry about except the sanctity of our souls.” This is not an argument about science, it is an argument about metaphysical possibility: I hold human extinction to be a possible outcome of climate change, while my opponent holds that Divine Provenience makes such an event impossible. How, then, do we make sense of modality?
One solution, and perhaps the most eccentric move in all of Analytic Philosophy, is to do what American philosopher David Lewis did, and propose modal realism. Lewis argued that there is an infinite number of real, really-real possible universes, containing every single variable and distinction possible. Thus, when we make talk of how things are possible, what we are doing is saying, there is a universe where it is the case that Jill Stein won the presidential election, where elephants live in the great plains of North Dakota, and where gravity is half as strong as it is in our world. These are all possibilities, and there is a world for them. Conversely, when we speak of impossibility (e.g. there are no four-sided triangles), we are saying there is no possible universe with a four-sided triangle, and so forth.
Modal realism is a very strange idea, and many philosophers dislike it because it is a very cluttered ontology, it seems to violate Occam’s Razor. Lewis would argue back that, although his theory is very quantitatively inflated, it is qualitatively reductive: instead of trying to make sense of modality in terms of psychology or fictions, modality is a fact.
Consider mathematics: many philosophers and mathematicians will accept that mathematical entities like numbers and sets are real things. They are part of the universe, and we are discovering rather than inventing them. Outer space aliens would come to the same mathematical theories as us. And yet, mathematical entities are utterly different than the sorts of entities that scientists study, they are non-empirical, non-causal, and seemingly non-temporal. If we can accept that there are different kinds of infinities, some larger than others, is it that much of a jump to say that an infinite number of causally-isolated universes?
Modality makes truth much more complicated. Consider news sources: a cable news channel can say, “Sources are telling us that the democratic party is rigged,” “Leading expert argues that household cleaners cause autism,” “Could [it be possible that] Ted Cruz be the Zodiac Killer?”None of these statements are strictly false, because the modal clause redirects their truth condition. This is how the media can spread outright misinformation without technically lying.
David Lewis is a controversial figure in contemporary metaphysics, and many people have challenged his flagship idea. However, Lewis vehemently defended his project, and offered perhaps one of the greatest lines in analytic philosophy against his critics:
“I don’t know how to refute an incredulous stare.”