C. B. Martin, "Dispositions and Conditionals," The Philosophical Quarterly, vol. 44, no. 174, January 1994, p. 1:
We must see that dispositions are actual, though their manifestations may not be. It is an elementary confusion to think of unmanifesting dispositions as unactualized possibilia, though that may characterize unmanifested manifestations.
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 rock is thrown at one, call it pane A, and it shatters. 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 possibilium. 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. The disposition is as actual as the thing that has it. Don't confuse a disposition with 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 Martin is right: It is an elementary confusion to think of unmanifesting dispositions as unactualized possibilia . . . .
The point extends to potentialities: it is an elementary confusion to think of unrealized or unmanifested 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.
The above must be borne in mind if we are to think clearly about the Potentiality Argument against the moral acceptability of abortion.