One of the more significant questions or obstacles to be dealt with when dealing with navigating between scientific and philosophical discussions of the multiverse, or in any discussion of the multiverse for that matter, is the task of figuring out whether or not the various sides of the discussion are even talking about the same thing.18 There are two critical distinctions between scientific accounts of the multiverse and philosophical accounts of the multiverse that will need to be clarified before any meaningful discussion can go on between the two. The first distinction to be made, one which contains a wide variety of further distinctions that must be made within it, is that of semantics. The multiverse discussion, unlike some other philosophical or scientific discussions, does not yet seem to have within it a precise and commonly accepted set of terms that carry with them exact definitions of how they ought to be used in various contexts. One of the most common cases of this is distinguishing between terms such as “universe” and “world,” with Tim Wilkinson noting that “[c]onfusingly, philosophers and physicists use ‘possible worlds’ and ‘possible universes’ interchangeably” (2013, p. 89). In some cases “universe” is taken to mean, and is used to refer to, the totality of all things in existence. That is to say; nothing can exist outside of the universe (except perhaps God, on some accounts, but this is not important for our purposes quite yet).19 In some cases with this usage of “universe,” “worlds” are considered to be smaller self-contained, often disconnected, sections within the “universe.” Each “world,” in this case, would exemplify a different way that things could have been, and collectively all of the “worlds” would compose the larger “universe.” On other accounts, it is the exact opposite, with the “world” being taken as the term to connote the totality of existence and a variety of “universes” that make up this world. With this in mind, it is not always altogether clear just how to compare and discuss various multiverse accounts that, while they may be using the same terms, will suffer from semantic incongruence. Similarly, various scientists and philosophers will coin their own terms to describe different levels of encompassment of the “worlds” or “universes” that they aim to discuss, so these too must be navigated and, in a sense, translated to undertake an accurate comparison among other accounts. Moving forward, the way that I will employ these terms will be as follows. When I speak of a multiverse, I am referring to a collection of universes (at least two) that, together, comprise the totality of all that is in existence. When I speak of universes, I am referring to spatiotemporally and causally disconnected units that comprise the multiverse. Universes are entirely self-contained and, for those beings and objects existing within their respective universes, represent the confines in which all causal relations can take place. And finally, when I speak of a world, I am referring to the totality of all that is in existence. In some cases, this will equate to a universe, and in some cases, it will equate to a multiverse, and this will vary depending on the particular ontological view being discussed. For a single-universe model, the world will consist of that single universe, whereas on a multiverse model the world will consist of all of the universes that form the multiverse.
Somewhat tied in with issues of semantics, and a potential influence of the differences that we see within the terminology of the discussion, are the motivations from which philosophical accounts and scientific accounts of the multiverse stem. On the one hand, scientific approaches to the multiverse are often concerned with justifying or delving into accounts of how the various universes that comprise the multiverse can come to pass in and out of existence. That is to say; scientific accounts are generally concerned with exploring the physical constants, laws of nature, mathematical structures, and the like, of these possible universes to determine whether or not their existence is even physically possible. Building on whether these individual universes are physically possible or not, the scientific approach will also go on to investigate by what process it is possible that these universes come into existence, what relation they may have to one another, whether or not it is possible that multiple universes exist at the same time, whether or not it is plausible for them to fit together into a multiverse, what the structure of this multiverse would be, and so forth. Scientists are generally concerned less with the contents of various universes than they are with the structure of the multiverse as a whole. They tend to focus purely on the aspects concerning the physical plausibility of different multiverse models.
Philosophical accounts of the multiverse not only concern themselves with different aspects of the multiverse than do scientific accounts but also begin with and are often motivated by very different goals. Philosophical multiverse accounts often tend to be driven by a desire to respond to various atheistic arguments. Philosophers can appeal to multiverse accounts in attempts to block various arguments for atheism, with these arguments for atheism generally being some version of the problem of evil or the problem of no-best-world. On the other hand, philosophers may also appeal to multiverse theories to undercut theistic arguments for God’s existence, namely, the fine-tuning argument. Given these motivations, the philosophical multiverse account is generally concerned not so much with determining how the universe came to be, as the scientific account is; instead it is more concerned with evaluating the overall value of various aspects of the multiverse. That is to say, evaluating the overall value of particular universes within the multiverse, or the multiverse as a whole, and determining whether or not certain universes ought to be considered as worthy of being part of the multiverse, or at least as being possible parts of it. It should be noted that the common employment of multiverse theories by the philosopher as a response to some objection can be seen as detrimental to itself, in some cases, with some philosophers seeing the multiverse theory as an ad hoc response to some particular argument. And that seems to be one of the more significant differences between scientific and philosophical multiverse accounts. While philosophical accounts are often mounted as a theistic response to deal with some particular atheistic objection to the existence of God (or as an atheistic response to some theistic argument for the existence of God), scientific accounts generally appear to be more independently motivated.
Conclusion
In this chapter, we have seen the differences between several philosophical multiverse accounts, as well as the differences between several different scientific multiverse accounts. Furthermore, we have seen that, while philosophical and scientific multiverse theories may come from different motivations and argue for different kinds of multiverses, the two disciplines do not operate independently of one another, with each side often taking and employing information and ideas from the other for their own theories. The result of this information and idea-sharing is a wide range of divergent multiverse theories, varying in terms of composition, value, makeup and origins, among other things.
Overall, this chapter aimed to present an overview of some of the prevailing multiverse views at work today and to demonstrate not only that there is a broad range of ideas at play but also that these views ought to be given serious consideration when considering the adoption of an ontological view.
NOTES
1. One particular area where this can be seen is in the discussion of actual infinites, which can be applied to whether or not a multiverse can contain an infinite number of universes rather than merely an extremely large finite number of universes. Some examples of work on infinites can be seen in Shapiro (2011), Tapp (2011), and Gabriele (2012).
2. For more in-depth survey pieces or additional discussions and accounts of the philosophical multiverse see Kraay (2012; 2015), Stewart (1993), Draper (2004), Forrest (1996), O’Connor (2008), Parfit (1998), and McHarry (1978).
3. I understand that this may appear to be an appeal to authority, but I make no claim regarding the acceptance of any claims regarding the multiverse. I merely want to point out that there is a substantial amount of serious work being done in the area and