Intention and Possibility
How does the idea of possibility enter our thought? It is generally agreed that perception is not its source: we do not see possibility—we don’t think of possibility because our senses present it to us. A more likely view is that imagination is the progenitor of the idea of possibility: it is because we imagine that the idea of possibility occurs to us. What is imaginable is possible and what is unimaginable is impossible. We find ourselves imagining things and therewith we introduce the idea of what could be. More strongly, we could not come up with the idea of possibility in any other way—imagination is our only access to the realm of the possible. Perception is certainly not, and what else is there? This theory has the result that a creature without imagination would be modally blind: it would have no use for the concept of possibility and no means of acquiring that concept. If the concept of the possible simply is the concept of the imaginable, then we cannot have the former without having the latter; but even if the connection is weaker, imagination is essential for the recognition of possibility.
The view is not without difficulties. Can we not imagine the impossible (water without H2O, disembodied minds)? And aren’t there possibilities that we can’t imagine simply because of our imaginative limitations (four-dimensional space, infinite time)? But I won’t discuss these well-worn issues; instead I will propose an alternative theory that I think has some merits. This alternative allows possibility to enter thought without benefit of the imagination, so that modal thinking can exist in the absence of imagination; the concept belongs in another matrix of interconnected concepts. The central concept in this matrix is intention: the concept of possibility is bound up with the concept of intention. This is because we can intend only what we believe or know is possible. If I believe it to be impossible to jump to the moon, then I cannot intend to jump to the moon. I can intend to do what I believe to be improbable, but not what I believe to be impossible. This is clearly true for logical or metaphysical impossibility, but it applies also to nomological impossibility: I cannot intend to do what I know to violate the laws of nature (as with jumping to the moon).
Thus when forming an intention I must be cognizant of possibility, implicitly or explicitly. This is a conceptual truth: it is conceptually impossible knowingly to intend the impossible. Accordingly, I cannot intend to intend what I know to be impossible, since that is itself impossible. To have the concept of intention is to know that intentions must concern the possible. If someone orders you to perform the impossible, you can rightly reply that you cannot form any such intention. You cannot be punished for failing to intend to do what you know it is impossible to do. This is simply beyond the power of your will. Thus the concept of intention embeds the provision that while you can intend a great many things you cannot intend what you know to be impossible. To have intentions is to be aware of this fact, and hence to have a grasp of the concept of possibility. It is not so for desire: you can desire what you know to be impossible, because desire does not have to reckon with the facts of the world. But intention is the intention to act and action perforce takes place in the world of possibility—hence you have to believe that your actions could realize your intentions. Even a cat gauging a difficult jump has to consider the possibility of success, and if it judges that success is impossible it will neither jump nor intend to. For reflective humans, understanding the nature of intention brings with it a grasp of possibility and impossibility. Thus we naturally contemplate possibility when we form intentions; we may even reason about possibility when deciding what to do.
But what should we say about more abstract and theoretical ideas of logical and metaphysical possibility? Here we can deploy the idea of a superior agent: logical impossibility is what God cannot intend to bring about. Even God could not intend to square the circle: his will is limited by his knowledge of modality. It isn’t that he can’t imagine doing this; it’s that he can’t intend to do it. And he knows he can’t intend it, whatever may be the case for his knowledge of what he can imagine. The impossible is that which cannot be intended—even by God. The concept of possibility is inextricably linked to the concept of intention, because intentions are modally sensitive—they must track what is possible and impossible.
The nice thing about this theory is that it locates modal concepts in something basic and practical. These concepts do not have to be seen as transcending the empirical world, as objects of pure intellectual apprehension; they arise from basic facts about agency.  We think this way because we are agents with intentions, and intentions must respect possibility. Any intention-forming agent must be sensitive to the modal facts: the limits of intention are the limits of the possible. The concept of the possible is the concept of what can conceivably be intended, while the concept of the impossible is the concept of what cannot conceivably be intended.
 These basic facts include the concept of ability—what an agent is able to do. No agent can intend to do what he knows he is not able to do, for one reason or another. So modal concepts have their roots in the concept of ability, which can be extended to different types of agent with different types of ability, up to the case of God. They are not quite as unworldly as they can be made to sound. In a certain sense abstract metaphysics has its roots in practical action.