Aristotle believed that the truthmakers for possibility statements were substances whose powers made it possible for those statements to be true. This belief came from his view of how things change. He thought that the only things that exist are substances and their states of having (or not having) certain properties and being (or not being) in certain relations (Pruss, 1996, p.25). Thus, change involves a substance going from one state to another. For example, a tree is a substance that can go from the state of having leaves to losing them, or from the state of being the tallest tree to being average in height. When the tree goes from one of these states to another, it changes.
Aristotle reasoned that change is only possible if there is a potential for change. Unless a substance has the potential to go from one state to another, it cannot and will not change. Unless a tree has the potential to lose its leaves, it cannot and will not lose them. But why do substances have these potentialities?
Aristotle argued that that potentiality is grounded in actuality. There is a potential for change only if it can be actualized, but a potential can only be actualized if a substance has the ability to undergo or bring about that change – otherwise change would not be possible (Pruss, 1996, p.35). Aristotle believed that the ability of a substance to undergo or bring about change is its power to do so. Actual powers are the basis for potentialities.
The powers of substances can be exercised in various ways. Some substances have a liability to exercise their powers inevitability and/or unintentionally in certain circumstances, as when a tree grows leaves under precise atmospheric conditions. Other substances exercise their powers intentionally, as when a gardener decides to hydrate the soil in which a tree is growing. In either case, a substance undergoes (or brings about) changes by virtue of its powers and those of other substances. How does Aristotle’s view supply truthmakers for possibility statements? Simply put, a statement is possible if a substance (or collection thereof) has the power to bring about its truth.
Aristotle’s account of modal truth is closely connected to what philosophers call “branching” theories of modality. A branch represents a possibility which is rooted in the capacities of a substance at a specific point in time. A single substance can be the root of many branches, and new branches can emerge depending on how substances interact at any given time – say, by restricting or sponsoring the activation of each other’s capacities. Branching theories are helpful because they visually represent specific points in history when propositions are made possible by the substances that exist at those times (Pruss, 1996, p.36).
 I will not presently discuss the possibility of powers that are completely unconstrained by other things – e.g. God’s – because the activation of a power normally depends on whether other substances are involved.
 It is less clear how Aristotle would ground statements about impossibility and necessity. Perhaps he would say that a statement is impossible if no substance has the power to bring about its truth; or necessary if no substance has the power to bring about the truth of its denial.