We appear to have (at least) two concepts of necessity, usually known as epistemic necessity and metaphysical necessity. Epistemic necessity concerns what could turn out to be the case—what might be true “for all we know”; it correlates with certainty (the Cogito is an epistemic necessity). Metaphysical necessity concerns what could really be the case—how things could be in themselves; it has to do with objective essence. The word “metaphysical” isn’t doing much work here: we could as well speak of non-epistemic necessity, since metaphysical necessity is defined by contrast with epistemic necessity. We could add analytic and nomological necessity to the list: what is conceptually necessary and what is necessitated by natural law. Standard examples of metaphysical necessity belong to neither of these categories, being both synthetic and modally stronger than nomological necessity. What is striking is that we have no analysis of metaphysical necessity, as we have an analysis of epistemic necessity. We can say that epistemic necessity is certainty and epistemic possibility is uncertainty (or ignorance), or we can analyze the concept in terms of epistemic counterparts; but we have nothing comparable to say about metaphysical necessity—here we have to take the concept as primitive. We have to take it as a brute fact that this table is necessarily made of wood or that a person necessarily has his or her actual parents. We have intuitions, but we have no account of these intuitions. This is quite puzzling: why should we have such intuitions, and where do they come from? Am I simply directly aware of the objective essence of things? Do I have a basic unanalyzable concept of non-epistemic metaphysical possibility? In the case of the other types of necessity we can see where they come from: from our state of knowledge, from concepts, or from the laws of nature. But metaphysical necessity appears ungrounded and unexplained: our concept of it appears primitive and inexplicable. This can fuel skepticism about the whole notion of metaphysical necessity (and possibility): is it perhaps just a trick of the imagination? What is its epistemology and what its conceptual underpinnings?
There is one form of modality we have not mentioned: what I will call agent modality. This concerns what we (and other agents) can and cannot do. What we are free to do is what we can do and what it is possible for us to do. We are aware of this kind of necessity and possibility from our own case, and we recognize it in others. We are, in fact, painfully conscious of the limitations on our possible actions, yet also conscious of what lies within our power. We can make comparative judgments about this kind of thing. We have the idea of beings with superior agential powers—God, in the extreme case. Thus I am now aware of my possible courses of action today, and of my life decisions (I could have been a psychologist instead of a philosopher). But I have no power to change my height or my species or my parents, and I know it. There are agential necessities as well as agential possibilities. These are not epistemic: it isn’t that I might turn out to be a psychologist after all, or that I am certain of the identity of my parents. Rather, these are objective facts about my powers of action—about my abilities. So here is a category of objective non-epistemic necessity to set beside the usual category of metaphysical necessity. Of particular interest is the ability to change things: I can change my location, my clothes, my hairstyle, and even my occupation; but I can’t change my parents or my species or my identity. So there is a correspondence between agential and metaphysical modality, and an affinity of nature. Is this a coincidence?
Consider Hesperus and Phosphorus: they (it) can change their location, but they can’t change their identity with each other. Planets have the ability to move, but they don’t have the ability to cease to be self-identical. Thus the concept of agential modality can be generalized to them: it isn’t a matter of free decision, to be sure, but it is a kind of power. Tables, too, can move, but they have no ability to change their material composition. Animals can walk around, choose a mate, and eat, but they can’t change their parental origin or species. Nor can other agents change the traits in question: it isn’t that we can change the identity of planets or the composition of tables or the origin of animals. No one can alter these things: they are agential necessities tout court. Not even God has the power to change these facts: he can’t make 3 even or water not H2O or Queen Elizabeth the daughter of Bertrand Russell and Gertrude Stein. God can do a lot—a lot is possible for God—but he can’t do just anything. Some actions are perfectly possible, within the agent’s powers, but some are impossible, even for the most powerful of agents. This is beginning to sound a lot like metaphysical modality, is it not? We might, then, venture a hypothesis: our concept of metaphysical necessity is an outgrowth of our concept of agential necessity (and similarly for possibility). We understand metaphysical modality on the model of agential modality—that’s where we get the idea from. We know what agential possibility is, originally from our own case, and then we generalize it to include metaphysical possibility. Accordingly, the examples of metaphysical necessity with which we are familiar are special cases of agential limitations, specifically limitations on God’s agency (or any conceivable agent). To be metaphysically necessary is to be such that no possible agent could change it. No possible agent could change this table from being made of wood to being made of ice—because that would make it a different table. You could replace each wood part with a similarly shaped chunk of ice, until the whole thing was changed to ice, but that would destroy the original wooden table, replacing it with a new ice table. Our intuition of necessity can thus be cashed out as an intuition of agential inalterability. That is what we are really thinking when we think that this table is necessarily made of ice: that no one could make it otherwise. This is not a conceptual reduction of the concept of metaphysical necessity (for one thing, it uses the concept of a possible agent); it is an attempt to link the unmoored concept of metaphysical necessity to something more familiar, more part of everyday life. It is a conceptual domestication—an elucidation or genealogy. It tells us from where the metaphysical concept derives. It tells us what family of concepts it belongs to, what its conceptual relatives are. It is true that the metaphysical concept transcends these practical origins, but it doesn’t entirely leave them behind: it builds on them, feeds off them, and exploits them. We might even offer that without them the concept of metaphysical necessity would not be available to us: we would draw a blank on questions of metaphysical modality if we had no prior notion of agential modality. The latter concept is a necessary precondition of possessing the former concept. It gives us the leg up we need. This is a case of conceptual leapfrogging or ladder climbing. Like many philosophical concepts, it takes its rise from something homelier.
We can test the hypothesis by asking how changeability correlates with necessity: are the least changeable things the things with the most metaphysical necessity? Numbers are notoriously changeless, but they are also heavily endowed with necessity: everything about them (almost) is charged with necessity. If we ask what can be changed about the number 3, the answer is hardly anything. By contrast, the self admits of a great many changes—of place, activity, psychology, perhaps even physical composition—and it is also highly contingent. Almost everything about the self is changeable and contingent: you can even in principle put the self in another body by brain transfer, and selves are not necessarily tied to a given body. The more a thing can be changed by a suitable agent the more imbued with contingency it is. Organisms and physical objects are intermediate between numbers and selves: quite a bit can be changed, but quite a bit can’t be. You can easily change the location of a cat, but not its body type (if you put a cat’s brain into a dog’s body, you don’t get a cat—though you may get the cat’s self). Tables will accept changes of location and color, but they resist being converted into TVs or repaired by being recast in a different material. This is all to say that our thoughts about what is metaphysically necessary or contingent are shot through with thoughts of what it is possible for agents to do. Two seemingly extraneous concepts thus intrude on these metaphysical matters: concepts of agents and actions. We are thinking of agents and we are thinking of their actions when we think about metaphysical modality. We aren’t just thinking of objects and their properties: we are thinking of what agents can and cannot do in relation to those objects and properties. When I think that I could have a had a different career I am thinking that I could have acted differently; when I think that a table could have been in a different place I am thinking of its powers of movement and of possible external causes of its movement (say, someone picking it up). When I think that I could not have had different parents I am thinking that, while I could have left my parents’ house earlier, it was not within my power to sever myself from them biologically. I am thinking, that is, of agency and action. My thought is not just about my possible properties, barely considered. Similarly, my modal thoughts about the table are not confined to the table and its properties; I am taking in other objects and other properties, specifically agents acting on the table. I am placing the table in a wider and richer conceptual context. So the concept of metaphysical necessity is not as bare and ungrounded as it may appear; it has its roots in a rather practical and useful set of concepts having to do with action. Epistemic necessity has its roots in concepts of knowledge, justification, and certainty; metaphysical necessity has its roots in concepts of agency, power, and action. Neither is self-standing and primitive.
 This is Kripke’s notion of epistemic modality in Naming and Necessity (1972): roughly, a situation is epistemically possible if we could be in an epistemic situation qualitatively identical to the actual situation and yet the facts are otherwise. It is notable that Kripke says virtually nothing to articulate the concept of metaphysical necessity, beyond noting (correctly) that it has a strong intuitive content. My aim here is to remedy that lacuna—so I am seeking to save metaphysical necessity not bury it. I want it to seem less strange. Less exotic.
 We can allow for grades of metaphysical necessity, according to how easy it is to change a given property. It is very easy to change one’s location, but not so easy to change one’s career or color or personality, so one is morepossible than the other. And that is intuitively correct: it does seem more possible to move to a different place than to acquire a different personality—since one condition is easier to achieve than the other. The binary opposition of metaphysical necessity and metaphysical contingency is too simple, too black and white. Similarly, epistemic necessity also admits of grades: some things are less epistemically possible than others—we can be more certain that the sun will rise tomorrow than that the stock market will rise tomorrow. Both types of modality come in degrees.
 Here is another point: the logical analogy between modal concepts and deontic concepts is well known, and deontic concepts concern agents and actions. Obligation maps onto necessity and permissibility maps onto possibility. Locating the source of modal concepts in agential concepts therefore comports with the general tenor of the concepts in question; certainly deontic modalities are explicitly agential.