The possible as opposed to the impossible is purely logical possibility, and so to the extent that we try to base the actual world on the possible one(s), we are not basing it even on a real possibility.

Logical possibility, or, the ability to be made in intellect. This is why STA defines it by predication.

1. #### E.R. Bourne said,

February 3, 2013 at 9:09 am

James,

In what way do we come to know real possibility? Is there an example of something that appears to be possible because it is logically possible but is actually impossible because it is, well, actually impossible? How do we arrive at such a conclusion?

• #### James Chastek said,

February 3, 2013 at 4:11 pm

That first epistemological question is extremely difficult from the POV of that we would want to deal with it. I’m terrified of even starting to answer it.

To the second question: it includes all things that are impossible by an unseen contradiction: “greatest prime number” or “a^n + b^n = c^n for n > 2″ might be examples. For examples from the physical world, tachyons suggest themselves, as does an animal of any given size (though we know that at some point it would get so large as to crush itself under its own weight). It might make sense to distinguish a “Logically possible to us” from a “logically possible to the wise” and both of these from a “logically possible to God” (would this match one-to-one with what was really possible? Why or why not?) In other words, we take the definition “impossible by an unseen contradiction” and then ask “unseen to whom?”

All this forces us to confront again your first question. How do we know when we have shown the real impossibility of Tachyons, animals of any given size, etc.? It certainly seems that if you say all real or physical objects are like this and can never be ruled out that you are committed to a Humean epistemology of some sort.

• #### Brandon Watson said,

February 4, 2013 at 7:34 am

I think one way to think of the distinction might be to think of logical possibility as not-ruled-out-by-available-information (i.e., none of the predicates on the table force us into contradiction). In that sense, we can make a lot of sense of relativity of logical possibility: not-ruled-out depends on what your ability to rule things out is. It’s possible (we can’t rule out!) that there might be impossibilities that can only be identified using predicates no human being could ever grasp adequately (too complicated, or, perhaps, too simple).

But if this is true, we shouldn’t assume that impossibility and possibility are symmetrical, either: not-ruled-out and ruled-out aren’t symmetrical. It would suggest that real impossibility is actually the primary reference point (or one of the two primary reference points, if we are counting actuality), and we think about real possibilities on the basis of it.

• #### James Chastek said,

February 4, 2013 at 1:29 pm

It would suggest that real impossibility is actually the primary reference point

This makes sense of something I was confused about. Logical impossibility is clearly logical as opposed to real (since, by definition, a logical impossibility cannot be real) and so is a better reference point for the logical as opposed to the real.

[W]e shouldnâ€™t assume that impossibility and possibility are symmetrical

That’s a good point. One of the arguments I wanted to put in the post but decided against was this:

The possible and impossible must be in one order
but logical impossibility is not real,
Therefore logical possibility is not real.

But the major premise is false, because it treats contradictories as though they were contraries, i.e as though possibility and impossibility had the symmetry of being in one order or genus. If I understand you, we should understand logical possibility as non-impossibility, where the “non” is negation. This would make logical possibility not-necessarily logical as opposed to the real.

• #### Mike Flynn said,

February 4, 2013 at 10:55 pm

Maybe an impossibility is a defectus in a possibility? Just as there is an asymmetry between life and death. There can be life without death, but not death without life (since the concept of death presupposes the prior existence of life). Just rambling, me.

2. #### thenyssan said,

February 3, 2013 at 10:47 am

I’m pretty sure a real possibility has to be observed as actualized.