In philosophy, modality is the study of possibility and necessity – of what is true, what could be true, and what must be true. When we talk about whether something is possible, impossible, or necessary, we’re talking about its modal status.
But not all kinds of possibility (or impossibility) are the same. In philosophy, people will usually distinguish between:
Logical possibility
Logical possibility is the broadest form of possibility. Anything that doesn’t contradict itself is logically possible, even if it isn’t possible in our actual world.
For example:
- Unicorns are logically possible
- A square whose sides are a trillion light-years in length is logically possible
- It’s logically possible that you could jump to the moon from Earth.
Conceivability is often linked with logical possibility – if you can coherently imagine an idea in your mind, that means such a thing is logically possible.
But if something is logically impossible it means it is self-contradictory, that one or more of the terms are logically incompatible.
For example:
- A 4-sided triangle is logically impossible
- A married bachelor is logically impossible
- 1+1=7 is logically impossible.
And again, this is linked to conceivability: The idea of a 4-sided triangle is inconceivable because it’s logically impossible – you can either imagine a 4-sided shape (a square) or you can imagine a triangle (3 sides), but you can’t coherently imagine a triangle that has 4 sides because the two things contradict each other.
Physical possibility
Physical possibility is the most narrow form of possibility. If something can be done given the laws of nature in our actual world, then it’s physically possible.
For example:
- It’s physically possible for a fish to breathe underwater
- It’s physically possible for a plane to fly
- It’s physically possible for a human to run 100 metres in under 10 seconds.
If something is physically impossible then it violates the laws of our physical reality – the laws of nature prevent it from being done. For example:
- It’s physically impossible for a human to jump to the moon from Earth
- It’s physically impossible to travel faster than light (assuming our current understanding of physics is correct)
- It’s physically impossible to drop an object on Earth and have it fall upwards (unless some other physical force intervenes).
The reason these things are physically impossible is because the laws of nature in our actual world prevent them.
But we can imagine a possible world where the laws of nature are different and so all these examples above are logically possible: We can imagine a parallel universe where the law of gravity is so weak that a human could exert enough force to jump, leave Earth’s gravity, and land on the moon – there’s no contradiction in that idea.
Everything that’s physically possible is logically possible, but not everything that’s logically possible is physically possible.
Metaphysical possibility
Metaphysical possibility is in between logical and physical possibility. It concerns what could be the case given the essential nature of things – what’s possible according to the way reality itself could have been.
But what exactly the ‘essential’ nature of things is is philosophically contentious – metaphysical possibilities are not as clearly defined as logical and physical possibilities.
An example: Water is H₃O instead of H₂O.
It’s logically possible that water could have the chemical structure H₃O instead of H₂O – “water is H₃O” is not self-contradictory in the same way “4-sided triangle” is.
But some philosophers argue that although “water is H₃O” is logically possible, it’s not metaphysically possible.
The reason they say this is because H₂O is an essential property of what water is – if something isn’t H₂O then it’s no longer water.
This essential property of water can be contrasted with the inessential or contingent properties of water, like boiling at 100°C. “Water boils at 222°C” is metaphysically possible because, in such a universe, water would still be water – it would just have a different boiling point. But a universe where water is H₃O is a step too far – at that point water would no longer be water and so it’s not metaphysically possible that water could be H₃O instead of H₂O.
So whether something is metaphysically possible or not depends on what it is – what its essential properties are. But whether a property of something is essential or contingent to it is up for debate. Some might say a human jumping to the moon from Earth is metaphysically possible, others might say it stretches the definition of ‘human’ too far and so is metaphysically impossible.
And a similar thing is going on with the zombie argument for property dualism: Property dualists say zombies are metaphysically possible, but physicalists often respond that zombies are not metaphysically possible because qualia are essential properties of certain physical/brain states.
But some philosophers reject the idea of metaphysical possibility altogether. They argue that there are only two meaningful kinds of possibility – logical and physical – and that talk of metaphysical possibility either reduces to one of these or adds unnecessary metaphysical baggage. On this view, once we’ve said something is logically consistent and physically impossible, there’s nothing in between – no further question about whether it’s ‘metaphysically’ possible.
Summary
- Modality is the study of possibility and necessity.
- We can define 3 ‘levels’ of possibility (but not all philosophers accept all 3):
- Logical possibility: What’s consistent with the laws of logic (i.e. anything that isn’t self-contradictory).
- Metaphysical possibility: What’s consistent with the essential nature of reality.
- Physical possibility: What’s consistent with the laws of nature in our actual world.
- These 3 kinds of possibility as nested:
- Physical possibility ⊆ Metaphysical possibility ⊆ Logical possibility
- Everything physically possible is also metaphysically and logically possible, but not everything logically possible is physically or metaphysically possible.
- Physical possibility ⊆ Metaphysical possibility ⊆ Logical possibility