Possibility and Actuality
How do possibility and actuality differ? Is there anything intrinsically different about them? Some metaphysicians have supposed that the difference is entirely extrinsic: actual states of affairs and possible states of affairs are intrinsically the same, but the former constitute what we call the actual world and the latter constitute possible worlds. Thus we have the indexical theory of actuality: the actuality operator is equivalent to the demonstrative “this world”. People in different worlds can employ this demonstrative, thereby treating their own world as actual. The worlds differ not at all in their intrinsic nature, with actuality just one instance of possibility; actuality is conferred from the outside, so to speak. Considered intrinsically, actual states of affairs and possible states of affairs are ontologically exactly alike. We could call this the modal uniformity thesis.
But consider the following fact: actual states of affairs always carry with them other actual states of affairs, while this is not true for possible states of affairs. For example, a wall is actually painted beige but could have been painted blue: its being beige is accompanied by being in a certain location, being a certain height, being painted by certain painters; but the possible state of affairs of being painted blue has no such accompaniments—it exists as an isolated fact. The possibility of being blue carries with it no commitments about what other possibilities combine with it: it could be blue and at any number of locations, of varying heights, painted by different painters, etc. Being possibly blue doesn’t cluster with other possibilities. By contrast, the actual state of affairs of being beige comes with a fixed totality of other actual states of affairs—there are no degrees of freedom here. Actualities come in packages not singly—in groups not individually. Once you actualize a possibility it loses its independence and becomes attached to other actualized possibilities. Actualities necessarily arrive in bundles, whereas possibilities exist in isolation from each other. We can call this the holism of the actual. If possibilities are atomic, actualities are molecular. Holism of the mental says that mental states necessarily come in bundles not in isolated singularities; holism of the actual says that actualities come in bundles not in isolated singularities. But possibilities are not subject to this kind of holism—they can exist in splendid isolation. We can envisage a possibility existing all by itself, but we can’t envisage an actuality existing all by itself—it must be embedded in a larger whole. If we think of actualization as a function, we can say that it takes separate possible states of affairs as its argument and gives as its value a complex of actual states of affairs. For example, the possible state of affairs of being painted beige yields as the value of the actualization function a package of multiple actual states of affairs. You can’t actualize being beige without actualizing a whole lot of other stuff, but you can create a possible state of affairs without creating other (logically unrelated) states of affairs. Actually being beige requires determinate other actual properties, but possibly being beige requires no such other properties—it exists as an isolated atom in modal space. It is a question how widely the holism of the actual extends—might it extend to the whole of the actual world?—but it certainly extends well beyond the actual state of affairs we are considering. It includes rather remote properties such as the material composition of the beige wall (actual walls always a specific material composition). By contrast, the mere possibility of being beige is quite neutral with respect to such extrinsic properties: it is not embedded in a determinate matrix of other possibilities—possible beige walls don’t have a unique material composition. Possibilities are lone operators (or only travel with close family) while actualities club together with other actualities (not necessarily logically related). Thus the modal uniformity thesis is false.
This means that there is an element of stipulation that characterizes merely possible worlds in contrast to the actual world. We don’t (and can’t) stipulate what actual facts coexist—that is a matter of how things actually are—but we do (and can) stipulate what possibilities combine with others. We say, “Consider a world in which pigs fly and horses talk”—and no one can stop us doing that—but we can’t say, “In the actual world pigs fly and horses talk”, because that is not actually the case. A possible world is made up of independent possibilities that are stipulated to coexist, but the actual world is made up of actualities that come in packages and which cannot be stipulated away. Possible worlds are like mental constructions in this respect, but the actual world is a given objective reality. When we announce that a world is a totality of states of affairs we must observe that the totalities are differently constituted, according as the world is actual or merely possible: in the actual world the states of affairs come glued together, so to speak, while in the possible worlds the constitutive states of affairs coexist by something more like fiat. We specify a possible world, but we discover the actual world. Actualization entails bundling, but mere possibility allows separation. So the principles of agglomeration are different for the actual world and possible worlds. Reality is more densely packed in the actual world than it is in possible worlds—that is, actualities are tightly bundled, while possibilities are free-floating (not counting inter-possibility logical entailments). Actuality is molecular; possibility is atomic. So actuality is intrinsically different from possibility: it adds something new to possibility. The ontological structure of the actual doesn’t simply recapitulate the ontological structure of the merely possible. Actuality is a different type of reality from possibility. It is not simply a matter of the indexical “this world”, or some other extrinsic view of actuality: the actual and the possible have a genuinely different mode of being. If we were to explore the universe of the merely possible, we would find a very different structure to reality there: the possibilities would be laid out in neat rows, carefully separated; they wouldn’t come in bundles, as they do in the actual world (a beige wall conjoined with a bunch of other properties such as a certain height, weight, and material composition). The world of possibilities is more like the world of stars, planets, and galaxies, structurally speaking—both are laid out in space without any interpenetration. Possibilities are like trees in a forest, flowers in a garden, children in a school. Actualities, on the other hand, are like units of technology or universities or branches in a tree—inherently holistic and cooperative. An actual state of affairs is always a sub-unit of something larger. A possible state of affairs, however, is self contained, free floating, not beholden to other states of affairs. Actualization alters the mode of existence of the possible by linking possibilities together into larger wholes: it is a process of assembly. It thus ends the self-isolation of the possible.
 This is the view associated with the work of David Lewis.
 The family members are just the logically related possibilities: the possibility of being both red and square always travels with the possibility of being red.
 From an abstract metaphysical perspective, the holism of the actual ought to strike us as more remarkable than we are apt to suppose. Actual reality is a kind of creative synthesis that contrasts sharply with the piecemeal nature of merely possible reality. The latter functions as a kind of disordered raw material for the former, mere ingredients for a would-be cake. Actualization is really a creative (almost miraculous) act, generating solid chunks of reality from languishing and idle elements. Possibility is like the formless gas that preceded the formation of stars and galaxies; actualization is like gravity in converting this unpromising stuff into something shapely and worth attending to. It is the holism inherent in actualization that makes reality as a whole interesting. If all we had were possible states of affairs, never actualized, reality would be a pretty sad and boring place; it is the actualization function that creates the world as we know it. Actualization is the root of everything worthwhile; mere possibility (pre-actualized reality) is a sorry business. Holistic actualization is the animating force of reality—the equivalent of divine creation. Without it possibilities are just aimless shut-ins going nowhere and communicating with no one. Being merely possible is a lonely and pointless kind of life; being actual is social and cooperative. When God was wondering whether to make possibilities into actualities he was wondering whether to inject reality with meaning (in one sense of “meaning”).