W.O. Quine's modal logic

Modal realism, as explicated by David Lewis in his work on the Plurality of Worlds, concerns the infinity of possibilities. On Lewis's view, there are infinite possibilities of worlds, where an infinite number of individuals are living an infinite amount of lives. This idea relates closely to the increasingly recognized concept of the "Multiverse," where more than one universe exist simultaneously. Perhaps the most surprising in Lewis's philosophy is the fact that he believes the existence of the possible worlds to be actual reality, rather than metaphors or philosophical concepts. Here, it is easy to become confused, as Lewis's idea of the reality of the possible worlds appear to dictate that they must exist in space and time somewhere. Indeed, some critics have gone as far as citing NASA's lack of evidence for life on other planets in the Universe to refute the philosopher's claim of infinity. It is by examining both critics and proponents of Lewis's thought that the philosopher's ideas come most poignantly to light, and by means of which a premise of one's own can be constructed.

Before such criticism is addressed, it is perhaps useful to consider Lewis's assumptions and premises, as offered in on the Plurality of Worlds. Perhaps the most interesting feature of Lewis's philosophy is that possible worlds other than our own exist in a reality that is as factual as our own. They also operate on the premise of certain laws, differing in content rather than kind. Furthermore, they are irreducible entities in their own right. Finally, when talking about the "actual" world of our reality, Lewis is of the opinion that such factuality is meant only in the indexical sense -- the "factual" world is meant to refer only to the premise that this is the world where we live, rather than any other possible world.

Lewis's work makes it clear that he intends his premises to be taken literally. In the first chapter of his book, the philosophers states:

…our world is but one world among many. There are countless other worlds… the worlds are something like remote planets; except that most of them are much bigger than mere planets, and they are not remote. Neither are they nearby. They are not at any spatial distance whatever from here. They are not far in the past or future, nor for that matter near; they are not at any temporal distance whatever from now. They are isolated: there are not spatiotemporal relations at all between things that belong to different worlds. (Lewis, 2001, p. 2).

From this, it is clear that Lewis does not intend his worlds to be taken as existing upon the same physical plane as our own. However, some critics believe that this is what Lewis meant by "something like remote planets." In reviewing on the Plurality of Worlds, Michael Brett for example cites a basic lack of physical evidence as reason to dispute Lewis's claim for plurality. Indeed, Brett appears to assume that the philosopher's stance is indicative of an age-old human wish to not be alone in the universe. It is a type of fantasy -- when looking skywards, the observer is comforted by the fact of infinite other worlds, where an infinite number of inhabitants resemble us, like the alternative realities of science fiction. Brett appears to take this premise in quite a literal sense, as Lewis indeed intends.

The basic problem is however that Brett appears to believe that these possible worlds are supposed to occur in the space-time continuum of the world that we consider as our reality. In keeping with this view, the reviewer cites well-known arguments against the existence of beings on other planets in the galaxy and the universe: the lack of physical evidence. Brett is convinced that such lack of evidence also means the lack of an infinite possible worlds.

He does argue convincingly that, since the start of space observation and exploration, not a single thread of evidence has been uncovered to suggest either the infinity of the universe or the existence of worlds other than our own. Indeed, his strongest argument is that, if the universe and possible worlds had indeed been infinite, surely the possibility of other worlds would have been far more evident than it is. He takes the lack of evidence in this regard to mean that there is no other possible world than the one that human beings in this reality experiences as such.

In examining these arguments, one must admit that Brett makes a good point. There are indeed no fossil records, ancient accounts, or indeed current evidence from our considerable technological searching devices to suggest that worlds beyond our own exist. Once again, however, evidence in this regard pertains only to one type of reality -- our own. The second part of the Lewis's assertion, as quoted above clearly indicates that the philosopher is not considering other possible worlds only in our universe and our reality. Instead, he appears to consider the existence of possible worlds as indeed being possible across a wide spectrum of realities. The fact that they are not part of our reality makes them no less real.

When the apparently paradoxical statement regarding other worlds is taken into account: "They are not far in the past or future, nor for that matter near," Lewis is clearly not referring to worlds that we can observe with physical scientific instruments. These worlds are apart from our own not only in physical reality, but also in terms of space and time.

The problem then becomes one of scientific deduction. How does Lewis know that these worlds exist? What are his premises for believing in the existence of such worlds? Lewis himself admits that he does not know very much about these worlds, nor how to find out more. He appears to base his philosophical system upon the observations of modality in our own reality. The fact that possibility, choice and there modality exists in our world and our language leads to the conclusion that other worlds must exist, where creatures such as ourselves make different choices or evolve differently to create a vast number of other worlds.

Each world is separate from the others in terms of space, time, and reality. However, the fact that we speak of our reality as the only one only means that it is the only one for us. We cannot access the other realities by means other than thought. There is no way to communicate with our counterparts in those worlds.

It is therefore not difficult to understand why critics such as Michael Brett so adamantly disfavors Lewis's premise. Indeed, Brett takes Lewis's premise quite literally, as apparently the latter intends him to. The main fallacy in Brett's observations is however that he applies it in terms of only a single reality. He does not or cannot accept, as Lewis apparently also intends the reader to, that realities other than this space and this time exist. He applies all Lewis's observations to a single reality, which then leads to the conclusion that Lewis must be incorrect.