universe instantiation eventually being realized at one point or another within this string of eternally self-reproducing universes. While Linde does not specify the exact process through which these individual universes come to pass in and out of existence (their lifespan, so to speak), he does make several suggestions that allow him to show that his model is consistent with various competing scientific theories regarding exactly just how such a process may be feasible.
The Intersection between Scientific and Philosophical Approaches to the Multiverse
As I have presented it, it may seem as if scientists and philosophers are working independently and in isolation of one another on matters of the multiverse, but this is not the case. Increasingly, philosophers are adopting and importing scientific work into their theories to both guide the arguments that they initially set out to make and serve as important tangible evidence for the cases that they go on to make. It is no longer the case that multiverse theories are relegated to pure speculation and loose inductive reasoning, instead now philosophers have substantial evidence from the scientific community to support their views regarding the nature of our existence, something that was once a far loftier ambition. That is not to say, of course, that the scientific views employed by philosophers are without controversy themselves. Just as various philosophical views face rival theories and conceptions, as we have just seen, so too do the scientific theories. We have just noted several different kinds of scientific theories that conclude in widely divergent types of multiverses, assuredly with each one being argued for by many experts in the field while simultaneously being argued against by many experts in the field. Scientific approaches to the multiverse will also sometimes borrow ideas from philosophical explorations, using various philosophical models to craft their hypotheses in the initial stages of experimentation to explore the possibilities and entailments of a multiverse. It is not often the case that experiments of this sort will begin with no aim; instead, there must be an idea present, something that is being searched for, an inquiry for which some answer is sought. These foundational inquiries are the things that philosophy can and does supply for the scientific world. Those aforementioned lofty philosophical projects serve as initial starting points for mathematicians, physicists and other scientists to begin their work, to aim to confirm or disconfirm some idea or thought. This is the role that philosophy plays in the science of the multiverse.
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.
