David Brightly comments:
The view I've arrived at is that sentences involving 'possibility' can be re-written into sentences involving just 'possibly', and that our modal notions arise from our encounter with inference. I'm happy to say, There is the possibility that the bulb will shatter -- we say things like that all the time -- provided it's understood to mean, Possibly, the bulb will shatter. I certainly don't want to commit myself to things called possibilities, unless they can be seen as constructions out of sentences, roughly, Possibly, S ≡ The truth value of sentence S cannot be determined from what we currently know together with deduction from known principles.
Can you persuade me otherwise? A 'big topic' I would imagine!
Let B be an ordinary light bulb. Light bulbs are typically fragile: they are disposed to shatter if suitably struck or dropped from a sufficient height onto a hard surface. I take Brightly to be saying two things. He is maintaining, first, that there is no more to the possibility of B's shattering in circumstances C than the truth of the sentence, 'Possibly, B will shatter in C.' Second, he is offering an analysis of 'possibly' in such sentences.
I take Brightly to be saying that there is nothing in B, and thus nothing in reality, that could be called B's disposition to shatter. In general, unrealized possibilities have no ontological status. But then what makes the sentence 'Possibly, B shatters in C' true? Presumably, Brightly will say that nothing makes it true: it is just true. He would not, I take it, say the same about 'B exists.' He would not say that nothing makes 'B exists' true, that the sentence is just true. I would guess that he would say that it is B itself, or perhaps the existence of B, that makes 'B exists' true. So there is something in reality that 'B' names, and this item is, or is part of, the truth-maker of 'B exists.'
But if he says this, should he not also admit that there is something in reality that make 'B is disposed to shatter in C' true?
To appreciate the point one must see that a disposition and its manifestation are different. B is disposed to shatter at every time at which it exists. But it needn't ever shatter. It might remain intact throughout its career. Therefore, the reality of a disposition cannot be identified with its actual manifestation. The same goes for powers and potentialities. If a man has a power he never exercises, it does not follow that he does not have the power. The potentiality of a seed to sprout in the right conditions is something real even if the seed remains on a shelf and its potentiality is never actualized.
There is an epistemological question that I want to set aside lest it muddy the waters. The question is: How does one know de re, of a particular light bulb, that it is disposed to shatter if it never does? I am not concerned here with the epistemology of modal knowledge, but with the ontology of the merely possible, which includes the ontology of unmanifested dispositions.
A disposition, then, is real whether or not it is ever manifested. But doesn't this just beg the question against Brightly? I maintain that unmanifested dispositions are real. Brightly denies this. If I understand him, he is eliminating unmanifested dispositions in favor of the truth of possibility sentences.
My objection to this invokes the Truth-Maker Principle: truths need truth-makers. Or at least many classes of truths need truth-makers, one of these being the class of truths about the powers, potentialities, dispositions, and the like of concrete individuals. (I am not a truth-maker maximalist.) My point against Brightly is that the sentence, 'Possibly, B shatters in C,' if true, is true in virtue of or because of something external to this sentence, namely, the unmanifested disposition in B to shatter.
My view is consistent with the view that unmanifested dispositions reduce to the so-called 'categorical' features of things like light bulbs. Unmanifested dispositions can be real without being irreducibly real. What I have said above does not commit me to irreducibly real dispositions. It commits me only to the reality of unmanifested dispositions, whether reducible or not.
" Possibly, S ≡ The truth value of sentence S cannot be determined from what we currently know together with deduction from known principles."
S in Brightly's example is 'The bulb will shatter.' True or false? I grant that the truth value cannot be known from what we currently know together with what we can deduce from known principles. But this cannot be what the possibility that the glass will shatter consists in. Brightly is making the very real possibility that the glass shatter, the bomb explode, the round fire, the cat scratch, Hillary throw a lamp at Bill, etc., depend on our ignorance. But then real possibility is eliminated in favor of epistemic possibility.
Suppose Sally knows that Tom is in Boston now and believes falsely that Scollay Square still exists. I ask Sally: is it possible that Tom is in Scollay Square now? She replies, "Yes, it is possible." But of course this is a mere epistemic possibility sired by Sally's ignorance. It is possible for all Sally knows. It is not really possible that Tom is in Scollay Square now given that the place no longer exists.
I don't think we should say that the possibility of the bulb's shattering consists in our igntrance as to whether or not 'The bulb will shatter' is true or false. Consider also that long before minded organisms arose in our evolutionary history, and thus long before there was knowledge or ignorance, there we seeds and such with real potencies some of which were actualized and some of which were not.