To say that something is necessary is to say that it must be the case. When something is not necessarily false, that is, when it could have been the case (including when it is the case, and even when it necessarily is the case), then we say that it’s possible. If something is neither necessarily true not necessarily false, then we say that it’s contingent.
Some comments:
The word “possible” in English (and related words like “might” and “can” and “must” — philosophers call these modal words) has different uses. If someone comes up to you on the street and says “It’s possible that I was born on the moon,” you’ll most naturally take him to be making some sort of (crazy) claim about what it’s reasonable for him to believe really is the case. You’d be understanding his claim as “For all I know, I was born on the moon.” This is a use of “possible” to express claims about evidence and knowledge. It’s called an epistemic sense of possibility.
But there is another thing the guy might have meant. He might have meant, “Look, I know I was born on the Planet Earth. But things could have gone differently. I could have been born on the moon — if for instance mankind had colonized the moon in the 1800s.” Here he’s using “possible” not in the sense of “for all I know…” but rather in a metaphysical or counterfactual or “alethic” sense. (“Alethic” comes from the Greek word for “truth.”) He’s not talking about what he knows or has evidence for. Instead, he’s talking about what could have happened, if the world or history had been different in certain ways.
Here’s a rule of thumb to help you out. If something of this sort is ever true:
then P is what we’ll call metaphysically possible. (Some philosophers say “logically possible.”) If nothing of that sort is true, then P is metaphysically impossible. There’s no way the world could be such that P would be the case.
We’ll be focusing on this second way of using words like “possible”.
What it means for something to be “metaphysically impossible” is that you’ve described a situation that is not really a way the world could have been.
Let’s try to come up with some things that would be examples of this.
Would it have been possible for the world to contain 3-sided squares? What would such a world be like? That doesn’t really seem to be a possible way for the world to be, does it?
Nor does it seem to be possible for it to be raining and not raining in the same place at the same time. Nor does it seem to be possible for a person to exist and fail to be one and the same person as himself. There are no possible situations in which things of those sort take place. They all seem to be metaphysically impossible.
Now, so far, the examples we’re coming up with of things that are metaphysically impossible all seem to have something to do with logic or definitions or math. That raises an interesting question:
This is something we’ll need to think carefully about.
Some harder cases:
With the last question we just asked, there are a few questions that differ in subtle ways, but that it’s important to try to keep apart.
One question is, does all the evidence and how things seem to you right now leave open the possibility that you don’t exist? Is it even possible to imagine everything seeming this way to you, but you not existing? Arguably the answer to this question is No.
If we express this by saying “It’s not possible that I don’t exist now,” we’re using “possible” in the “epistemic” sense mentioned in point 1, above.
A second question is, can you imagine some other time or place (or some other way for the world to be) where you don’t exist? On the face of it, this does seem to be something I can do. I can imagine Ancient Rome, for example, without me being there. Some philosophers argue that this isn’t really achievable after all. They argue that when you try to imagine something, you really have to imagine yourself there observing it. These are interesting issues, but we’re not going to pursue them. If you want to read/think more about them, here is a transcript from lectures by Shelly Kagan, a philosopher at Yale, discussing arguments about this.
A third question is, would it be possible (in the metaphysical or counterfactual sense) for you to have never existed? On the face of it, this also seems to be possible. You might think it’s possible because you can imagine it happening (contra the philosophers mentioned a moment ago). Or you might think that even if you can’t imagine it, still you can understand how it could have happened. If the conditions at the start of the universe had been slightly different, maybe the Earth would never have coalesced where it did and gathered oxygen and so on, and in that case, there would have been no you.
When I asked “Would it be possible for you to have never existed?” I meant to be asking this third question.
Here’s how some jargon/terminology we encountered before connects to what we’re discussing now.
If it’s metaphysically necessary that a thing have certain properties, then we say that those properties are essential properties of that thing, and that they are part of the thing’s nature or essence. So for instance, oddness is one of the essential properties of the number 3. Some philosophers would argue that being made of wood is one of the essential properties of my table. If they’re right — if it is an essential property of the table — then there would be no possible situation in which the table exists, but it’s made of steel. (There may be possible situations in which some other table exists in the same location, and is made of steel. But if being made of wood is essential to this table, then the steel table would have to be a different table. It couldn’t be one and the same table as this table.) If you agree with those philosophers, you might also incline towards thinking it wouldn’t have been possible for you to have been made of snow.
Those properties which are not essential, we call accidental properties. For instance, being 3 feet tall is only an accidental property of the table (we could saw part of its legs off to make it shorter).
As we said before, this is a specific technical sense of “accidental,” which of course diverges from the ordinary folk meaning of the phrase. Consider an athlete who eats an excellent careful diet and exercises for hours every day. In the philosopher’s sense, this athlete’s being in good shape is an accidental property — she would still be the same person if she stopped being in good shape. But of course, it’s not a coincidence that she’s in good shape. It’s not an “accident” in the ordinary folk meaning of that phrase.
As we also said before, for philosophers, talk about your “essential” properties is not the same thing as talk about your most important or most valuable properties. It is talk about properties which it is impossible for you to lose. Your wit and charm may be among your important properties. But it’s possible for a person to become un-witty, and un-charming, while still being the same person. So wit and charm are not essential properties.
When we were thinking about examples of things that might be metaphysically possible or impossible, we raised the question:
When philosophers talk about “definitions,” they distinguish between:
Some examples of impossibilities we might consider seem like they’d only need to violate a scientific definition, not a conceptual definition. For example, it doesn’t seem to be part of the conceptual definition of water that it be H2O. But arguably it wouldn’t be possible to have water without having H2O. It doesn’t seem to be part of the conceptual definition of being you that you be made of flesh and blood and bones rather than snow. But arguably it wouldn’t be possible for you to be made of snow.
We’ll consider more examples of this as we proceed.
In our Zoom meeting, I gave a more complex example where we can get metaphysical impossibilities that aren’t just the result of conceptual definitions.
To begin, we need to distinguish between claims about language and claims about the world.
Suppose there are two women, Beverly and Helen. And suppose I’m madly in love with Beverly, but she doesn’t know I exist. Helen on the other hand is in love with me, but I don’t like Helen. What I want is to be loved by Beverly. How could I get what I want?
Would getting Helen to change her name to “Beverly” help me get what I want? Of course not. Even if Helen changed her name, she would still be Helen, and Beverly would still be Beverly. It doesn’t matter what names we call them. What I want is to be loved by a certain person, Beverly, not just to be loved by somebody or other whose name is “Beverly.”
Similarly, a stone would still be a stone, even if there were no people. It would still be a stone even if no one used the word “stone” to talk about it. It would still be a stone, though it would not be called “a stone.” It would still be a stone, even if people called it by other words, like “dog.”
“Jim Pryor” is a name for me, the person I actually am. Now, could I fail to be Jim Pryor? Well, there might be possible situations in which I have a different name, and so in which I’m not called “Jim Pryor.” There might be possible situations in which other people are called “Jim Pryor.” But I’m not interested in those situations. I’m not interested in what people might be called.
Nor am I interested in the question whether I might inhabit a different body; or whether I might live a different life (for example, becoming a spy instead of a philosopher).
“Jim Pryor” is a name for a particular person, not a name for some human body or biography. What I want to know is: could there be any possible situations in which I’m not that person? Any situations in which that person, Jim Pryor, the person who is actually called “Jim Pryor,” is somebody other than me?
It does not seem like there could be. For there to be such a situation, I would have to be somebody other than the person I actually am. And that is not possible. The person I am can not possibly be identical to something other than himself.
It may be possible for me to live somebody else’s life, and to be called by somebody else’s name. For instance, perhaps I could have lived the life of Napoleon, and have been called “Emperor Napoleon,” instead of “Jim Pryor.” But even if that were to occur, and I experienced all the events of Napoleon’s life, it would still be me, this person, Jim Pryor, the person who is talking to you right now, experiencing them.
So now we have one result:
And yet, in certain circumstances, I would be able to imagine or conceive of myself existing, while disbelieving that I am Jim Pryor, and even while imagining that Jim Pryor does not exist. For instance:
So our second result is:
I can conceive of such a world, but as said a moment ago, such a world is not genuinely possible. Hence, the mere fact that I can conceive of something does not guarantee that that thing is genuinely possible.
Many philosophers used to think that if you can conceive or imagine something without confusion or contradiction, that proves that it’s possible.
Nowadays, philosophers realize that you have to be more careful. There is no contradiction in the amnesiac professor’s conception of himself existing but Jim Pryor’s not existing. At least, there is nothing there that goes against any conceptual definitions he has for “I” or “Jim Pryor.” He knows how to use those terms perfectly well, and no amount of pure reasoning or logic will enable him to figure out that they name the same person.
So the fact that you can conceive or imagine something without any conceptual contradiction can at best be some reason or evidence that the thing is possible. It can’t be an absolutely certain proof. Some things can be conceived of that are not possible, but rather necessarily false. One only manages to conceive them because one is ignorant in certain ways. They are necessarily false, but one does not know that they are false.
The case of the amnesiac professor is an example of that. The amnesiac professor does not know that he is Jim Pryor. So he does not know that wherever he goes, there Jim Pryor goes. It’s nonetheless necessary that wherever he goes, there Jim Pryor goes. What’s necessary and what’s possible for him depends on which object he is. It does not depend on what people (or he himself) know about him.
Work on these kinds of examples is very important these days in philosophy of language and philosophy of mind.
Trying to determine which of the things we can conceive/imagine really are possible, and which aren’t, is a very challenging problem in contemporary debates about the mind/body problem.