In Naming and Necessity, Kripke gives a number of examples of essential properties in order to show that not all necessities are a priori or analytic. He is not concerned to develop a general metaphysics of modality, a systematic classification of necessities and possibilities. But that project is a worthwhile one, and relatively unexplored. I shall offer some remarks on it, hoping to show that there is some interesting structure here: there are patterns and generalizations. I won’t re-defend Kripke’s examples (most of which have sources elsewhere) but take them as given; my question is what general picture they promote. Thus I will accept that there are necessities of origin, kind, and composition: a given human being, say, essentially has the parental origin she has; she is essentially of the kind human; and she is essentially composed of certain biological materials (cells, carbon, etc.). No one could be this human being and not have those properties. These are metaphysical necessities concerning individual human beings. I couldn’t have been born to the British royal family or been a dog or be made of glass—though perhaps someone looking like me could have these properties. By contrast, certain properties of human beings are contingent and could easily be lacked without detriment to identity: I could have had a different occupation or lived in a different place or never pole-vaulted. It would still be me, just living a different life. My history is contingent, but my origin, kind, and composition are not.
Well and good: but is there any deeper story to tell? Do we just have a series of examples of essential and contingent properties with nothing to unify them, or might there be something in common to the examples? Is there a principled dualism or just a list of unrelated instances? With respect to essential properties, I think we can accept two important points. The first is that the list we have so far is complete: Kripke didn’t omit an important class of necessary truths. He never claimed completeness, but reflection suggests that he found it—there are no other de re necessities waiting to be recognized. True, we can analyze the relation of origin and detect various necessities of origin (parents, sperm and egg, strands of DNA); and true, we can distinguish necessities of composition that relate to types as well as to tokens (this table is necessarily made of wood, the type, and also necessarily made of this particular piece of wood, a token)); true also, we can distinguish human beings from persons and accordingly raise two different questions about necessities of kind. But there doesn’t seem to be any additional category of de re individual essence that has not been mentioned; our list appears exhaustive (there is surely no necessity regarding bodily organs, for example, since one can be given someone else’s kidney and have an artificial heart implanted).
The second point is that the three categories extracted from Kripke’s text are logically independent of each other: none entails the others. Thus we can’t deduce origin from natural kind or composition from origin. We have three distinct types of necessity here, not reducible one to the other. This is true even if we extend essentialism beyond biological entities, claiming that individual atoms, say, have necessities of origin, kind, and composition: this very hydrogen atom couldn’t have come from anywhere but the big bang (that event) or been an iron atom or be made of anything but quarks. We seem to have run the gamut—that’s about it as far as essence is concerned. Where an object came from, what it is made of, what kind of thing it is—that exhausts its essential nature; everything else is contingent. We might thus declare a triune theory of individual essence—a holy trinity of separable types of necessity. It would have been nice to find a deeper unity, but it turns out that 3 is the magic number—at least it wasn’t 7 or 29! The three essences do seem naturally connected, certainly not opposed to each other, but there is no apparent way to unify them into a single attribute. Hence we can announce the doctrine of Threefold Essence.
It might be supposed that contingency will yield a richer harvest of types. Aren’t there hugely many kinds of contingent property—occupation, location, hobbies, prejudices, talents, acts performed, things owned? Where is the unity here? The class of contingent truths appears to be hopelessly heterogeneous, a mere motley. But I think, perhaps surprisingly, that this is wrong: there is really only one kind of contingent truth! Or better, all kinds of contingent truth have the same unitary basis. Consider states of motion: being at rest or traveling through space. Suppose I am at rest now, sitting quite still: I could have been in motion, pacing around, playing tennis, driving my car. It is entirely contingent what my state of motion is at any given time. The same is true for any physical object: its state of motion is a contingent property of it—it could exist and yet be in a different state of motion. In fact, if you wanted to give a clear and convincing example of a contingent property, you couldn’t do better than to pick motion—motion is the paradigm of the contingent. An object’s motion is not part of its intrinsic nature, what makes it what it is. Intuitively speaking, motion belongs to the career of an object, not its constitution–its behavior not its being. Maybe an object’s potential for motion is written into it, but its actual state of motion is just so much adventitious history—alternative motion is easily imaginable. Even when motion follows strict laws of nature, as with elliptical planetary motion, we can easily conceive it being otherwise: the earth is not what it is in virtue of tracing ellipses around the sun instead of circles. Just as the earth is not necessarily inhabited, so it is not necessarily in elliptical orbit about the sun.
But what about other types of contingent property, say being a philosopher? They are not types of motion. True enough, but notice that motion is involved in their coming to obtain: I became a philosopher by taking a particular path through space, acting in specific ways, moving my hand to write philosophy essays, etc. I came to have the property of being a philosopher by virtue of certain motions (some in my brain). The same thing is true of my more athletic attributes, as well as musical. So I think we can venture this generalization: every contingent property of an object supervenes on motion. Nothing happens but that motion makes it so. The property might not be a state of motion, but its instantiation depends on certain motion properties being instantiated. When I imagine myself not being a philosopher I imagine various motions not having occurred (e.g. moving from Manchester to Oxford in 1972). So what unifies the class of contingent properties is their dependence on motion—which is the paradigm of the contingent. Whenever we conceive of certain properties not holding we conceive of enabling motions not occurring. This is the basis of our sense of contingency. There is really only one kind of contingent truth—the kind that depends on episodes of motion. History is the history of movement, ultimately. Contingency is therefore monistic, tracing back to a fundamental kind of contingency. Necessity comes in three irreducible types, but contingency is always the same. The loose relation between objects and space is the ground of contingency.
Immediately we notice that origin, kind, and composition have nothing to do with motion. They imply nothing about how things move. They are not part of a thing’s dynamic history—what happens to it or what it does. It is a necessary condition of a property being essential to a thing that it not be a motion-dependent property, but it also seems to be sufficient for essence that the property not involve motion. Any truth about an object that does not directly or indirectly relate to its motion is a necessary truth about it. Take color and shape: these are not essential properties, to be sure, and they seem static, but don’t they tacitly involve motion—motion of parts or particles? The shape of an object might be constant for a period of time, but apply appropriate forces and you get movement of parts—hence the shape is contingent, since it can be altered by motion. Color is contingent because it can be changed by the passage of light coming from the object and by the tiny motions of receptors responding to the incoming light. When we imagine shapes and colors being otherwise we imagine certain motions occurring or not occurring. But no change of motion in an object can change it from having the origin it has to having a different origin, and similarly for kind and composition. Properties are essential when and only when they don’t involve motion, and they are contingent when and only when they do.
This is a pleasing generalization, but can things really be that simple? Does the modal structure of the world divide up so neatly? Consider numbers: motion is not involved in their having the properties they have and all their (intrinsic) properties are essential. The number 2 is essentially a number, is essentially even, is essentially the predecessor of 3, is essentially a divisor of 16, etc. It has no history that could have been otherwise, no movement that we could imagine reversed—no location, job, hobby, or talent. Nothing happens to it and it does nothing. Movement is alien to its being. Thus it is all essence. It is the same with geometrical figures: they participate in no marches or street-crossings and possess no moveable parts. Contingency is accordingly not in their nature. Contingency enters the life of an object only when history comes to visit, but history consists of motions large and small. In other words, contingency feeds on events, and where there are no events there is no contingency. Then all is necessity. A purely platonic world would lack contingency because nothing would happen in it that could have been otherwise. Universals track no paths through space that they might not have tracked. No journey, no contingency.
There is a line of objection to our neat binary picture that one seldom hears urged today, though it is not without precedent, namely that there is no real contingency in the universe. Everything that happens happens by necessity. This is the opposite of the modal skeptic who denies that anything is really necessary (except maybe analytic truths). Suppose determinism is true, so that everything that happens follows from the laws of nature and hence is nomologically necessary. Suppose too that we regard nomological necessity as a form of metaphysical necessity. Then we reach the conclusion that everything must have happened as it actually happened: there are no contingent facts. Granted, there are impressions of contingency, but these turn out to be illusory upon closer analysis—they confuse what is (allegedly) contingent for this object and what might be true of some counterpart object. We are familiar with the idea that what seems contingent for a natural kind is really what is possible for some other natural kind similar to the one in question (e.g. some liquid similar to water might not be H2O but not water itself). Well, according to the metaphysical view we are considering, when it seems to us that an object might have been otherwise in some respect we are really thinking of some other object that might be that way. In fact, all objects simply play out their essential nature in their actions and reactions (Leibniz held a view like this). If so, all facts are necessary facts–we are merely under an illusion of contingency. I don’t say this view is correct, only that it intelligibly has the consequence that necessity is ubiquitous. In effect, it takes motion to be the necessary unfolding of the intrinsic nature of the universe—though we may not be able to grasp the way this unfolding works. Indeed, it can be maintained that only a view like this can render the world intelligible, since pure contingency is unintelligible (it violates the principle of sufficient reason). If reason is built into the universe, it must work by rational principles, but these can only be necessary truths. Motion, in particular, cannot be arbitrary and spontaneous; it must be written into the nature of things. The world may appear to harbor a deep contingency but this is just an appearance—underneath it has a rational order. I had to become a philosopher; it wasn’t just an accident that could have been otherwise. States of motion are essential properties after all.
I mention this metaphysical position for the sake of completeness, not to endorse it. The position that seems right to me is the usual binary one: we have essences and we have accidents. The essences revolve around origin, kind, and composition, while the accidents owe their existence to the nature of motion. We can grant that motion is governed by natural laws that carry their own type of necessity, and hence strong determinism is true, but that doesn’t add up to full metaphysical necessity. We can conceive these laws being otherwise in a way we can’t conceive origin, kind, and composition being otherwise. There are metaphysically possible worlds in which I became a quantity surveyor and was born in Australia (my parents emigrated) but not worlds in which I am a tiger or made of glass or came from an acorn. The basic structure of modal reality is thus a triad of essential properties, on the one hand, and a unified class of motion-dependent contingent properties, on the other. There is nothing more and nothing less.
 I don’t mean to assert dogmatically that no other necessities will ever be discovered, though that may be true; I mean only that I don’t know of any obvious ones that fail to show up in Kripke’s text.
 I won’t discuss whether all natural objects exhibit all three types of essence, animate and inanimate, but I am inclined to think it is true.
 As an exercise in astronomical essentialism we can ask what the necessary properties of the earth are. First its origin: it necessarily came from the stuff it actually came from (probably a bunch of celestial dust); second its kind: it is necessarily a planet; third its composition: it is necessarily made from a specific collection of assorted elements. The earth (that object) couldn’t have come from some other source; it couldn’t be an elephant; and it couldn’t be made of jelly. But there might be a planet that looked like earth but had a different origin and composition (and maybe was a living organism).
 Events and time are different: a given event couldn’t have occurred at a different time, e.g. WWI occurring in 1963 (though there could have been a similar war at that time). Whether objects can exist at other times is a difficult question: could I have been born in 1940 or 1066?
 Isn’t it a contingent property of the number 2 that it is the number of my cats? We can talk that way, but notice that the alleged property is relational not intrinsic; indeed, it is entirely extrinsic to the number. It is not part of the nature of 2 that it numbers my cats—not a truth of arithmetic.
 Kripke toys briefly with this idea in Naming and Necessity, p.99.
 Modality is more streamlined than we might have supposed, less variegated. God had relatively little to do in creating necessity and contingency compared to creating all the truths. When creating all the possible worlds he followed a few simple precepts. Reality is modally parsimonious.