Take the fragility of a piece of glass. Its fragility is its potentiality (passive potency, disposition, liability) to shatter in certain circumstances. Consider two panes of thin glass side by side in a window. The two panes are of the same type of glass, and neither has been specially treated. A small rock is thrown at one, call it pane A, and it shatters under the moderate impact. The other pane, call it B, receives no such impact. We know that A is fragile from the fact that it shattered. ("Potency is known through act," an Aristotelian might say.) We don't have quite the same assurance that B is fragile, but we have good reason to think that it is since it is made of the same kind of glass as A.
Suppose that B never in its existence is shattered or in any way pitted or cracked or broken. Then its fragility, its disposition-to-shatter (break, crack, etc.) is never manifested. We can express that by saying that the manifestation of the disposition remains an unactualized possibility. That is, the shattering of pane B remains, for the whole of B's existence, a merely possible state of affairs, a mere possibility.
But that is not to say that the disposition is a mere possibility, let alone that it is unreal. The disposition is as actual as the thing that has it. A disposition is distinct from its manifestation. The disposition is actual whether its manifestation is actual, as in the case of pane A, or merely possible, as in the case of pane B.
So we make a distinction between the (de re) possibility of B's shattering and B's disposition to shatter. The first is the possibility of the manifestation of the second. The first may never become actual while the second is as actual as B. What's more, the possibility of B's shattering is (in some sense needing explanation) grounded in B's disposition to shatter.
The point extends to potentialities: it is an elementary confusion to think of unrealized or unmanifested or unactualized potentialities as unactualized possibilia or mere possibilities. For example, a human embryo has the potentiality to develop, in the normal course of events, into a neonate. This potentiality is something actual in the embryo. It is not a mere or unactualized possibility of the embryo. What is a mere possibility is the realization of the potentiality. Just as we must not confuse a disposition with its manifestation, we must not confuse a potentiality with its realization.
One difference to note is that between a passive potentiality and an active potentiality. The pane's potentiality to shatter is passive whereas the embryo's potentiality to develop into a neonate is active. As for terminology, I don't see any non-verbal difference between a potentiality-to-X and a disposition-to-X. (I could be wrong.) Some people are irascible. They are disposed to become angry under slight external provocation. Is that a passive potentiality or an active potentiality? Put that question on the 'back burner.'
2. Another difference between a possibility and a potentiality is that, while every actual F is a possible F, no actual F is a potential F. Therefore, a possible F is not the same as a potential F. For example, an actual cat is a possible cat, but no actual cat is a potential cat. A towel that is actually saturated with water is possibly saturated with water; but no towel that is actually saturated with water is potentially saturated with water. If a man is actually drunk, then he is possibly drunk; but an actually drunk man is not a potentially drunk man. Potentiality excludes actuality; possibility does not. But can't a man who is actually drunk at one time be potentially drunk at another? Of course, but that is not the point.
Necessarily, if x is actually F at time t, then x is possibly F at t. But, necessarily, if x is actually F at t, then:
a. It is not the case that x is potentially F at t
and
b. X is not potentially F at t.
Furthermore, an actual truth is a possible truth, but it makes no sense to say that an actual truth is a potential truth. A truth is a true proposition; propositions are abstract objects; abstract objects are not subjects of real, as opposed to Cambridge, change. So it makes no sense to speak of potential truths.
The actual world is a possible world; but what could it mean to say that the actual world is a potential world?
If God necessarily exists, then God actually exists, in which case God possibly exists. But it makes no sense to say that God potentially exists. In terms of possible worlds: If God exists in every world, then he exists in the actual world and in some possible worlds. But 'God exists in some potential worlds' makes no sense.
It makes sense to say that it is possible that there exist an individual distinct from every actual individual. But it makes no sense to say that there is the potentiality to exist of some individual distinct from every actual individual.
3. So, to answer Vlastimil's question, potentiality is not to be confused with possibility. And it doesn't matter whether we are talking about narrowly logical possibility, broadly logical possibility, nomological possibility, institutional possibility, or any other sort of (real as opposed to epistemic/doxastic) possibility. Nevertheless, the two are connected. If it is possible that a boy grow a beard, then presumably that possibility is grounded in a potentiality inherent in the boy. The point, once again, is that this potentiality is not itself something merely potential, but something actual or existent, though not yet actualized.
I am now seated. I might now have been standing. The first is an actual state of affairs, the second is a merely possible state of affairs. How are we to understand the mere possibility of my standing now? Pace the shade of David Lewis, it would be 'crazy' to say that there is a possible world in which a counterpart of me is standing now. But it seems quite sane to say that the possibility of my standing now, when in actual fact I am seated, is grounded in the power (potentiality) I have to stand up.
A mere possibility is not nothing. So it has some sort of ontological status. A status can be secured for mere possibilities if mere possibilities are grounded in really existent powers in agents.
('Potential' Puzzle. I have the power to do X iff it is possible that I do X. But do I have the power because it is possible, or is it possible because I have the power? Presumably the latter. But my power is limited. What constrains my power it not what is antecedently possible? Throw this on the 'back burner' too, Euthyphro!)
As I understand the Aristotelian position, real possibilities involving natural items are parasitic upon causal powers and causal liabilities ingredient in these items. That, by the way, implies constituent ontology, does it not? Score another point for constituent ontology.
The Aristotelian position also implies a certain anti-empiricism, does it not? A rubber band that is never stretched never empirically manifests its elasticity; yet it possesses the dispositional property of elasticity whether or not the property is ever manifested empirically. So dispositions and potencies are in a clear sense occult (hidden) entities, and they are occult in a way the occult blood in your stool sample is not occult. For the latter, while not visible to gross inspection is yet empirically detectable in the blood lab.
4. Go back to the two panes of glass. One we know is fragile: it broke under moderate impact. How do we know that the other is fragile? I submit that the concept of potentiality underlying the Potentiality Argument is governed by the following Potentiality Universality Principle:
PUP: Necessarily, if a normal F has the potentiality to become a G, then every normal F has the potentiality to become a G.
To revert to the hackneyed example, if an acorn is a potential oak tree, then every normal acorn is a potential oak tree, and this is so as a matter of natural necessity. It cannot be the case that some normal acorns have, while others do not have, the potentiality to become oak trees. Potentialities are inherent in the things that have them. They are not a matter of ascription. We don't ascribe potentialities; things have them regardless of our mental and linguistic performances. And these very performances themselves realize potentialities. So if the potentialities of the ascribing mind were themselves ascribed, who or what would do the ascribing? I cannot ascribe potentialities to myself if the ascribing is itself the realization of my potentiality to ascribe.
Similarly with passive potentialities. To say of a sugar cube that it is water-soluble is to say that, were it placed in water, it would dissolve. Now if this is true of one normal sugar cube, it is true of all normal sugar cubes. Suppose you have 100 sugar cubes, all alike. There would be no reason to say that some of them are water-soluble and some are not. If one is, all are. If one is not, none are.
5. Note that the water-solubility of sugar cubes cannot be identified with the truth of the subjunctive conditional 'If a sugar cube were placed in water then it would dissolve.' It needs to be identified with the truth maker of that conditional, namely, the passive potency to dissolve inherent in the sugar cube.
6. Potentiality as here understood brings with it further Aristotelian baggage.
Pointing to a lump of raw ground beef, someone might say, "This is a potential hamburger." Or, pointing to a hunk of bronze, "This is a potential statue." Someone who says such things is not misusing the English language, but he is not using 'potential' in the strong specific way that potentialists — proponents of the Potentiality Principle in the Potentiality Argument– are using the word. What is the difference? What is the difference between the two examples just given, and "This acorn is a potential oak tree," and "This embryo is a potential person?"
The difference is explainable in terms of the difference between identity and constitution. A lump of raw meat cannot come to be a hamburger; at most it can come to constitute one. The same goes for the hunk of bronze: it cannot come to be a statue; at most it can come to constitute one. Note also that an external agent is required to shape and cook the meat and to hammer the bronze. An acorn and an embryo, on the other hand, can come to be an oak tree and a person, respectively, and indeed by their own internal agency. Potentiality in the strong sense here in play is therefore governed by the following Potentiality Identity Principle:
PIP: Necessarily, if x is a potential F, and there is a y such that y realizes, whether partially or fully, x's potentiality to be an F, then x = y.
Note that PIP does not imply that there is a y that realizes x's potential. Potentialities, after all, may go unrealized similarly as dispositions may go unmanifested. A seed's potential will go unrealized if the seed is destroyed, or if the seed is not planted, or if it is improperly planted, or if it is properly planted but left unwatered, etc. What PIP states is that if anything does realize x's potentiality to be an F, then that thing is transtemporally numerically identical to x. So if there is an oak tree that realizes acorn A's potentiality to be an oak tree, then A is identical over time to that oak tree. This implies that when the acorn becomes an oak tree, it still exists, but is an oak tree rather than an acorn. The idea is that numerically one and the same individual passes through a series of developmental stages. In the case of a human being these would include zygote, embryo, fetus, infant, child, adolescent, and adult.
Not so with the hunk of bronze. It is not identical to the statue that is made out of it. Statue and hunk of bronze cannot be identical since they differ in their persistence conditions. The hunk of bronze can, while the statue cannot, survive being melted down and recast in some other form.
Consider the Pauline verse at 1 Corinthians 13:11: "When I was a child, I spake as a child, I understood as a child, I thought as a child: but when I became a man, I put away childish things." This implies that numerically one and the same man, Paul of Tarsus, was first a child and later became an adult: it is not as if there was a numerically different entity, Paul-the-child, who passed out of existence when Paul-the-adult came into existence.
So not only is potentiality (in the strong Aristotelian sense here in play) governed by PIP, it is also governed by what I will call the Potentiality Endurantism Principle:
PEP. Necessarily, if x is a potential F, and there is a y such that y realizes, whether partially or fully, x's potentiality to be an F, then x (= y) is wholly present at every time at which x (= y) exists.
PEP rules out a temporal parts ontology according to which a spatiotemporal particular persists in virtue of having different temporal parts at different times.
Let me throw another principle into the mix, one that is implicit above and governs active potencies. I'll call it the Potentiality Agency Principle:
PAP. Necessarily, if x is a potential F, and x's potential is to any extent realized, then the realization of x's potential is driven, not by any agency external to x, but by x's own internal agency, with the proviso that the circumambient conditions are favorable.
The notion of (strong) potentiality that figures in the Potentiality Principle and the Potentiality Argument is governed by PUP, PIP, PEP, and PAP at the very least.
7. When Barack Obama was a community organizer was he 'potentially' president of the U. S.? It was surely possible that he become POTUS: logically, nomologically, and institutionally: there is nothing in the Constitution that ruled out his becoming president. And there is nothing incorrect in saying, in ordinary English, that the young Obama was 'potentially' POTUS. But does it make sense to say that, ingredient in the young Obama, there was a potentiality that was actualized when he became POTUS if we are using 'potentiality' in the Aristotelian sense?
I don't think so. It looks to be a violation of PUP above. Let 'F' stand for U. S. citizen. Does every U.S. citizen have the potential to become a presidential candidate? Obviously not: it is is simply false that every normal U. S. citizen develops in the normal course of events into a presidential candidate. A potentiality is a naturally inherent nisus — and as natural not a matter of laws or other conventions — which is the same in all members of the class in question. But the opportunity to become president has nothing natural about it: it is an artifact of our contingent laws and political arrangements. People like Obama do not become presidential contenders in the way acorns become oak trees.
