There are at least three areas of inquiry traditionally in the purview of philosophy that now are often claimed to be best — or only — studied scientifically: metaphysics, epistemology, and ethics. Let us discuss each in turn.
Physicists Stephen Hawking and Leonard Mlodinow open their 2010 book The Grand Design by asking:
What is the nature of reality? Where did all this come from? Did the universe need a creator? ... Traditionally these are questions for philosophy, but philosophy is dead. Philosophy has not kept up with modern developments in science, particularly physics. Scientists have become the bearers of the torch of discovery in our quest for knowledge.
Though physicists might once have been dismissive of metaphysics as mere speculation, they would also have characterized such questions as inherently speculative and so beyond their own realm of expertise. The claims of Hawking and Mlodinow, and many other writers, thus represent a striking departure from the traditional view.
It is perhaps in part as a response to this apparent consilience that we owe the rise of a large professional and popular literature in recent decades dedicated to theories about multiverses, “many worlds,” and “landscapes” of reality that would seem to restore the lack of any special favoring of humanity. Hawking and Mlodinow, for example, state that
the fine-tunings in the laws of nature can be explained by the existence of multiple universes. Many people through the ages have attributed to God the beauty and complexity of nature that in their time seemed to have no scientific explanation. But just as Darwin and Wallace explained how the apparently miraculous design of living forms could appear without intervention by a supreme being, the multiverse concept can explain the fine-tuning of physical law without the need for a benevolent creator who made the universe for our benefit.
The multiverse theory holds that there are many different universes, of which ours is just one, and that each has its own system of physical laws. The argument Hawking and Mlodinow offer is essentially one from the laws of probability: If there are enough universes, one or more whose laws are suitable for the evolution of intelligent life is more or less bound to occur.
Though these arguments may do some work in evading the conclusion that our universe is fine-tuned with us in mind, they cannot sidestep, or even address, the fundamental metaphysical questions raised by the fact that something — whether one or many universes — exists rather than nothing. The main fault of these arguments lies in their failure to distinguish between necessary and contingent being. A contingent being is one that might or might not exist, and thus might or might not have certain properties. In the context of modern quantum physics, or population genetics, one might even assign probability values to the existence or non-existence of some contingent being. But a necessary being is one that must exist, and whose properties could not be other than they are.
Multiverse theorists are simply saying that our universe and its laws have merely contingent being, and that other universes are conceivable and so also may exist, albeit contingently. The idea of the contingent nature of our universe may cut against the grain of modern materialism, and so seem novel to many physicists and philosophers, but it is not in fact new. Thomas Aquinas, for example, began the third of his famous five proofs of the existence of God (a being “necessary in itself”) with the observation of contingent being (“we find among things certain ones that might or might not be”). Whether or not one is convinced by Aquinas, it should be clear that the “discovery” that our universe is a contingent event among other contingent events is perfectly consistent with his argument.
Writers like Hawking, Mlodinow, and Smolin, however, use the contingent nature of our universe and its laws to argue for a very different conclusion from that of Aquinas — namely, that some contingent universe (whether or not it turned out to be our own) must have come into being, without the existence of any necessary being. Here again probability is essential to the argument: While any universe with a particular set of laws may be very improbable, with enough universes out there it becomes highly probable. This is the same principle behind the fact that, when I toss a coin, even though there is some probability that I will get heads and some probability that I will get tails, it is certain that I will get heads or tails. Similarly, modern theorists imply, the multiverse has necessary being even though any given universe does not.
The problem with this argument is that certainty in the sense of probability is not the same thing as necessary being: If I toss a coin, it is certain that I will get heads or tails, but that outcome depends on my tossing the coin, which I may not necessarily do. Likewise, any particular universe may follow from the existence of a multiverse, but the existence of the multiverse remains to be explained. In particular, the universe-generating process assumed by some multiverse theories is itself contingent because it depends on the action of laws assumed by the theory. The latter might be called meta-laws, since they form the basis for the origin of the individual universes, each with its own individual set of laws. So what determines the meta-laws? Either we must introduce meta-meta-laws, and so on in infinite regression, or we must hold that the meta-laws themselves are necessary — and so we have in effect just changed our understanding of what the fundamental universe is to one that contains many universes. In that case, we are still left without ultimate explanations as to why that universe exists or has the characteristics it does.
When it comes to such metaphysical questions, science and scientific speculation may offer much in fleshing out details, but they have so far failed to offer any explanations that are fundamentally novel to philosophy — much less have they supplanted it entirely.