Este important, pentru scopuri de natur etic, s se spun ce putea s se fi fcut, dar nu a fost fcut, i ce s-ar fi ntmplat dac s-ar fi fcut. Este general acceptat c ntr-o oarecare msur a posibilului, o fiin uman poate fi responsabil pentru un act dac a fost mcar logic posibil s l fi evitat. Atunci cnd spunem c valoarea moral survine n aciunile i circumstanele non-morale, spunem c nu se putea s fie cazul ca valoarea moral a cuiva s fie diferit dei aciunile i circustanele non-morale erau aceleai.Atunci cnd discutm problema rului, uneori ne ntrebm dac este posibil ca Dumnezeu i rul s coexiste, o problem diferit de ntrebarea de facto dac rul din lume face existena lui Dumnezeu probabil sau nu.Atunci cnd discutm despre obiecte natural, des nu putem specifica n care categorie se ncadreaz fr s vorbim despre proprieti dispoziionale, adic proprietile care ar fi realizate dac circumstanele ar fi altele. Ceva ar putea tri, de fapt, toat viaa sa ca un cal, dar e adevrat c dac ar fi nghiontit sub burt, unde de fapt nu a fost niciodat nghiontit, deodat i n mod natural ar face aripi i i-ar lua zborul, nu este un cal.Capabilitile noastre de expresie ar fi cu foarte mult srcite fr poate fi, ar putea fi, trebuie s fie, e posibil, e necesar, ar fi. Avem nevoie de aceti termini ca s vorbim despre realitatea din jurul nostrum. Totui, paradoxal, discuiile care implic posibilitatea deseori nu par s fie niciodat despre ceva real. Unicornul care este posibil nu exist, nu am procedat altfel dect am fcut-o i aciunile i circumstanele non-morale sunt aa cum sunt.Un mod de a conceptualize noiunile modale este s ne gndim la o lume posibil, un fel (cu fel neles suficient de larg nct s nu prejudiciem ntrebarea ontologic despre ce lumi posibile exist)n care un cosmos ar fi putut s fie. Lumi posibile diferite sunt diferite feluri n care lumea noastr ar fi putut fi.Principala alternativ de a ne gndi la o modalitate n acest sens global este s ne gndim la ea ntr-un sens local, s ne gndim la cai alternative prin care poriuni din lumea aceasta ar fi putut fi. O aa modalitate, fragment cu fragment, este ceea ce limbajul obinuit utilizeaz. Atunci cnd spuneam c Hitler ar fi putut s nu se fi nscut niciodat, nu spunem, de obicei, doar c exist o lume posibil n care el s nu fi existat de exemplu, o lume n care universal este i ntotdeauna a avut o desitate a energiei constant neschimbtoare. Vrem s spunem c fragmentul din lumea aceasta, care corespunde cu naterea lui Hitler ar fi putut s nu existe, dei mult din marea majoritii a restului lumii, n special cel puin trecutul ndeprtat d dinaintea naterii lui Hitler era, n mare, la fel, iar legile naturii erau aceleai pe care le avem. Ce nseamn exact s fii inut fix n aceast ar fi putut s nu se fi nscut aseriune depinde de context. Aadar, n timp ce apparent vorbim doar de fragmente de lumi, contextual determin despre ce lumi ntregi vorbim, mai prcis ce fragmente din lumea actual ar trebui s fie imaginate ca ramnnd n acel scenario posibil n care Hitler nu s-ar fi nscut niciodat. Ca s facem mai puin ambigu discuia noastr despre posibilitatea fragment cu fragment, aducem in calcul noi lumi posibile.Nevoia de a discuta despre lumi ntregi este artat n mod special clar atunci cnd facem afirmaii contrafactuale. Pentru c suntem obinuii s ntrebm lucruri de genul Cum ar putea sau ar fi fost cursul istoriei daca Hitler nu s-ar fi nscut? i ntr-o not plauzibil despre cum s rspundem la astfel de ntrebri, ar trebui s ne gndim la lumi ntregi n care Hitler nu s-ar fi nascut, i s spunem ce funcioneaz n astfel de lumi. Avnd n vedere ce fixeaz contextual nostru, adic majoritatea evenimentelor de dinaintea naterii lui Hitler i legile nature, putem spune anumite lucruri despre ce se petrece n acele lumi. De exemplu, cursul evenimentelor n alte galaxii este la fel ca i n lumea prezent - indiferent ca evenimentele oribile din secolul al XX-lea s-au petrecut sau nu nu va afecta ce se petrece n alte galaxii, chiar i doar pentru c informaiile despre ele, cltorind cu viteza luminii, nu au ajuns nc aici. Dar cursul istoriei locale ar fi fost mcar puin diferiti putem specula despre cum ar fi putut fi diferit. Uurina n a spune dintr-o rasuflare c evenimentele din alte galaxii ar fi fost la fel dar evenimentele de aici ar fi fost diferite indic ntr-adevr c este potrivit s analizm situaiile contrafactuale holistic.Mai mult, ce este posibil ntr-un fragment de lume poate foarte bine depinde de restul lumii respective. De exemplu, ce se petrece categoric aici theoretic depinde de ce legi ale naturii sunt rspunztoare. Este imposibil s existe o lume cu legi ale naturii fr excepii ca ale noastre dar unde lucrurile nu cad atunci cnd sunt scpate n circumstanele potrivite; totui, n afar de astfel de legi, este cu siguran posibil. Este imposibil s existe ru nejustificat ntr-o parte a universului dac exist o deitate atotputernic, atottiutoate i bun n univers.

Moreover, when there is such a deity, then what evils can exist in a portion of the world may well depend on what happens elsewhere in the world, since the justification of some evil in one portion of the world may depend on events elsewhere. Our ordinary modal claims need to be contextually disambiguated, and when thus disambiguated are seen as involve whole possible worlds. Because of all this, possibility and necessityprima facierequire reference to be made to whole possible worlds, and so one should try to make sense of possible worlds. In conversation, Rescher has objected that the holistic intuitions apply to physical (or at least, I suppose, causal) and not necessarily logical possibility. However, at least we should leave open the option that logical necessity might involve some holistic aspects. For instance, suppose that the essentialist intuitions are correct that something could not be water were not certain laws of nature in place. Then the claim that it is possible that, say, it is possible for the water in a glass to fail to be gravitationally attracted to the center of the earth might be aglobalclaim for it is a claim that there could exist certain counterfactual laws of nature, which could be global ones, and that water could logically co-exist with these laws. (Note, by the way, that the truth value of this claim is unclear.) Of course maybe laws of nature will turn out not to be global, but then specifying that they lack global modal oomph will itself be a global claim. Given a basic notion of possible worlds, whatever their ontology, we need some correlative notions. By the (or our) cosmos I shall mean the aggregate (i.e., mereological sum) of all actually existing things. By the (or our) universe I shall mean the aggregate of all actually existing spatio-temporal things. Each world represents or corresponds to a way the cosmos could have been. In what way this representation works is open at this point of the investigation. One of the worlds shall be distinguished as the actual world, i.e., the world that represents the wayourcosmos in fact, or actually, is. An individual exists in a worldwif, were that world actual, that individual would exist, or, equivalently, ifwrepresents the cosmos as containing that individual. A proposition is true at a worldwif, were that world actual, that proposition would be true, or, equivalently, ifwrepresents the cosmos as satisfying that proposition. Occasionally, the term domain will be used for the collection of all possible individuals that exist in a given world. What the notions of represents, actual, exists in and true at really signify will depend on what our ontology of possible worlds is. There are many possible such ontologies. There is the crazy one, which nonetheless will be conceptually useful at times to keep in mind, that there necessarily is a Platonic library somewhere which contains physical books, of infinite size, each of which gives a maximal consistent description of a cosmos in some fixed language.[3] On this view, a world is one of these books. A worldrepresentssome possible way of being a cosmos if the book that the world is describes the way that cosmos would be correctly. A world isactualif everything written in it is true. A proposition istrue ata world if it is expressed by some sentence in the book. An individualexists ina world if the world describes the individual as existing. Other theories will have other renderings of the basic notions. For instance, David Lewis thinks that all possible ways that the universe could be is a way that some concretely existing universe really is. Moreover, cosmoi and universes are the same for him. Thus, worlds are concrete universes. A world represents some cosmos if itisthat cosmos. Theactual worldis the world we inhabit. A proposition istrue ata world if it truly describes a state of affairs obtaining in that world. An individualexists ina world if it inhabits that world. A propositional Ersatzist may take a world to be a maximal collection of compossible propositions. Theactual worldis the collection all of whose propositions are true. A world corresponds to a cosmos by having as its members propositions true of that cosmos. A proposition istrue ata world if it is a member of it. An individualexists ina world if some proposition in that world says that the individual exists. Leibniz, on the other hand, thinks that worlds are maximally consistent ideas in the mind of God. The actual world is the idea that God has chosen to actualize. An idea corresponds to a universe by being a mental representation of it. A proposition istrue ata world if it is a part of, or maybe represented by, that world. An individualexists ina world if the idea represents him as existing. We can now give a possible worlds semantics for possibility and necessity claims. It is possible thatpproviding there is a worldwat whichpis true. It is necessary thatpprovidingpis true at every world. Having possible worlds lets us consider local and global modalities in a uniform way. When I say Hitler might not have existed in an ordinary way, I am saying that the proposition that Hitler does not exist is true at some world which matches ours in various relevant respects. When I say It is logically possible that unicorns exist, I may just be making the claim that the proposition that unicorns exist is true at some world, without putting any restriction on which worlds are relevant here. Some further terms will be useful. A proposition iscontingentproviding it is true at some but not all worlds, i.e., providing neither the proposition nor its negation is a necessary truth. An individualxis anecessary beingif it exists in all worlds. An individual is acontingent beingif it exists at some but not all worlds.