Throughout the history of philosophical reflection, one thought has been at the forefront of man’s consideration: the nature of the universe. From Aristotle to Leibniz, questions about the world’s existence and the foundations of reality have been the driving force for deep intellectual inquiry. Significant progress (particularly from current modes of science) has led modern man to the realization that the universe does not have to be the way that it is. Fundamental particles could have been different from the way they are. The laws of nature could have taken different values than the ones they have. There might have been very different particles and laws altogether; indeed, no universe at all. It seems that nothing about the universe’s structure dictates its necessity—in other words—it is a contingent entity. It seems likely, then, that there be a cause of the universe’s existence that explains its unnecessary state of affairs. Something that its contingent composition is dependent on, and that is itself not contingent.
But this brings up a more fundamental question—do contingent events require an explanation of their contingency? This assumption, known as the Principle of Sufficient Reason (PSR), is one that seems intuitively self-evident and undergirds all of causal inquiry. If a state of affairs is not necessary, then why does it obtain? Philosophers Alexander Pruss and Josh Rasmussen affirm this intuition stating that “…in the absence of defeaters, one has some degree of justification [a priori] for accepting the explanatory principle.” The search for explanations (Why is the light on? Who left this here? Why are you crying? Why is it raining?), indeed, the whole of scientific investigation is predicated on the idea that contingent events have explanations beyond those events themselves.
But some think the PSR claims too much too quickly. For one, it is a sweeping metaphysical principle that cannot be shown to be wholly extensive—there may be exceptions to the rule. Perhaps some or even one contingent event needs no explanation. In fact, quantum physics seems to open the door for such exceptions to the principle. Indeterministic interpretations of quantum mechanics show that virtual particles spontaneously materialize out of the quantum vacuum without any deterministic or antecedent explanation. Specifically, when it comes to the very early universe, quantum physics is its descriptor. Such unexplainable phenomena may be responsible for the universe’s contingency.
The inability to demonstrate the PSR to be coextensive with all of reality does give one pause. It is difficult to conclude that there are not (or cannot be) exceptions to the rule. However, this should not undermine the principle, or it’s general applicability to reality. Most principles, whether physical or metaphysical, cannot be shown to be wholly extensive. This fact does not mean the principle should be dismissed or its application withheld. If there is an exception to the principle, then there must be something about the type of contingent event that makes it an exception—and this can be analyzed.
Maybe that exception is found in quantum mechanics as suggested from some interpretations of quantum theory? Certain quantum phenomena seem unexplainable in terms of an antecedent physical description. Such events may not be necessary, yet they remain unexplainable. But are such events unexplainable? It seems that quantum phenomena are, in a very strong sense, dependent upon the quantum vacuum, physics, and laws. If there were no vacuum or laws, there would be no quantum event. In that way, the explanation of quantum events is found in the existence of the quantum vacuum and physics. Perhaps the question could be asked, “why do quantum vacuums and laws cause these phenomena?” This is admittedly a more difficult question to answer. Maybe it is a necessary feature of quantum states of affairs to produce unpredictable phenomena, and therefore, are not contingent events. Or perhaps the PSR needs to be stated in a softer way that leaves room for events like those found in quantum physics. A plausible revision might be to say that “contingent existence (as opposed to events) requires an explanation.”
There is a by-product of the PSR that some may find undesirable—it implies the existence of a necessary being. If all contingent existence has an explanation beyond itself, then there must be a kind of existence that finds its explanation within itself. Otherwise, an infinite regress of explanations occurs. Given the intuitive warrant and lack of defeaters for the PSR, it seems plausible to accept the implication that at the foundation of existence, there is a necessary entity. What about the universe? Is it possible that the universe is this necessary reality?
Two features of the universe make it unlikely to be the necessary entity implied by the PSR: the scientific affirmation of a possibly different fundamental structure, and the evidence for its beginning. If the universe does not have to exist in the way that it does, or, indeed, could have failed to exist, then this affirms its contingent nature. The current standing of philosophical and scientific thought seems to affirm both of these features.
The modern understanding of fundamental physics demonstrates that different kinds of universes could have been instantiated. Fundamental particles could have been different from the way they are. The laws of nature could have different values, and energy quantities could have been greater or lower. These possibilities don’t just show the flexibility of the physical world, but their non-essential nature. According to the Historian of Science and skeptic Michael Shermer, even if there is necessary existence, it doesn’t imply the necessity of “our universe with our particular laws of nature that give rise to atoms, stars, planets, and people.”
Another astonishing discovery from both philosophy and modern science is the universe’s termination at a beginning point. Medieval philosophers (in particular, the Muslim theologian al-Ghazali) have argued for hundreds of years against the possibility of an infinite regress of past events.  If such a regress were able to be actualized, then the number of past events could never be traversed. For example, if the number of past events is infinite, then how has today arrived? At any point prior to today, there would always be an infinite number of events to traverse in order to arrive at today. This suggests that if the past were infinite, today would be unreachable.
Admittedly, when it comes to the nature of infinity, concepts begin to become less clear. The Australian Philosopher Graham Oppy has offered counter-examples aimed at showing the regularity of the traversal of an actual infinite. He gives one such example of a ball bouncing on slabs with a decreasing time-interval between each bounce. Oppy says, “If the ball hits the first slab at one minute to twelve, the second slab at ½ minute to twelve, the third slab at ¼ minute to twelve, and so on, then the ball can come to rest on a slab at twelve, having made infinitely many bounces…” Here Oppy demonstrates a task (a ball’s bouncing) that has an infinite number of events and yet is completely traversable. This counter-example calls into question the example from the universe’s past events.
The bouncing ball example from Oppy is a so-called “supertask” because of the balls exponentially increased speed at which the task is completed. The example is thought-provoking but seems to be sneaking in a conceptual difference. When thinking about an actual infinite number of past events, it is the event-intervals themselves that are said to be infinite. No matter how you divvy up past events (minutes, hours, seconds, ½ seconds), there is always an infinite number of them. Not so for the bouncing ball example. It is only by dividing the measurement itself that you can approach infinity, but any pre-picked measurement (½ minute, ¼ minute, etc.) will still be finite. So, in Oppy’s example, between 11:59 and 12:00, there is only one minute, two half-minutes, four quarter-minutes, and so on. But for the universe’s past events, there are infinite minutes, infinite half-minutes, infinite quarter-minutes, etc.
This suggests that Oppy is not comparing apples to apples here, but, instead, is demonstrating the potentially infinite division of any measurement and applying that division to the event itself—hence “supertask.” Nothing in his example provides a similar comparison to that of an actually infinite number of events. Even so, the eternality of the past is not like that of a “supertask.” If such a supertask could be completed, this does not give reason to think the past number of events was completed in the same way. So the philosophical argument against a past-eternal universe seems to stand.
The recent discoveries of modern science have corroborated these philosophical insights. Cosmology tells us that space and time have an origin point at a singularity. When scientists track the expansion of the universe back in time, they see space-time come back to a point of infinite density, before which, there is nothing.  If this is correct, then the universe’s contingency would be exemplified at its absolute origin point.
More recently, The Director of the Institute of Cosmology at Tufts University, Alexander Vilenkin, helped to develop the Borde-Guth-Vilenkin (BGV) theorem, which states “that if the universe is, on average, expanding, then its history cannot be indefinitely continued into the past.” Vilenkin does not expect a physical loophole; he states, “The BGV theorem is sweeping in its generality. It makes no assumptions about gravity or matter.” He is clear—the theorem’s equations imply that there must be a past, space-time boundary. If the BGV theorem is correct, then this seems to be compelling evidence that cosmological models pointing to a space-time origin are, in fact, accurate.
Some scientists have tried to construct models of the universe that will avoid the BGV theorem and, therefore, its implications. One such model is an eternally closed, static universe that “burst into inflationary expansion” at some spontaneous time. According to the theory, the universe would not be in an on-average state of expansion because—having existed for an infinite amount of time—expansion would be just an infinitesimally small part of the universe’s lifespan. Vilenkin shows that such a universe would be susceptible to unstable quantum phenomena. Given an infinite amount of time, no matter how unlikely, a quantum collapse would occur. Vilenkin’s argument from the quantum instability of this model seems almost philosophical. If the universe had existed for infinite time and then burst into expansion, why hadn’t it already done so at some previous point, given the existent probability of quantum collapse and an infinite amount of time? Apart from this, the physicist Aaron Wall’s work on thermodynamics shows that, even at the quantum level, the Second Law implies a singularity, which implies a beginning.
A second model some physicists suggest is a universe that has been cycling through an infinite number of expansions and contractions. According to Vilenkin, these models have typically been unable to avoid a beginning, given the conservation of entropy between each of its cycles (as described by the second law of thermodynamics). Some physicists have tried to avoid this by suggesting the expansion period be longer than the contraction period. Vilenkin points out that if expansion is longer than contraction, then this puts the universe in an on-average state of expansion throughout its lifetime. This implies its subjection to the BGV theorem. It seems that Vilenkin accurately shows that both static and cyclical models of the universe cannot avoid a beginning, given the boundary described by the BGV theorem.
The current scientific evidence points in one direction—the universe had a beginning. If the universe could have failed to exist (as suggested by the science), then we have a strong reason to believe that the universe is not, in fact, necessary in its existence. If the revised PSR (that contingent existence requires explanation) holds, then we have a warranted intuition to believe that the universe is explained by something other than itself.
Given the practical and a priori intuitions about the PSR, the flexibility of the laws of nature, and the current standing of modern cosmology, it is likely that the universe has an external cause. The PSR is supported by intuitive notions of inquiry and is the driving force of science. It has never been falsified (not even by quantum physics) and provides good grounds for thinking that a contingently existent entity should be explained. The evidence from science and philosophical reflection makes it probable that the universe had an absolute beginning. Cosmological models attempting to prevent the beginning are not likely, and the BGV theorem (along with the Second Law of Thermodynamics) makes the prospects of avoiding a beginning low.
It seems reasonable to conclude that, given the contingent nature of space-time, the universe is dependent upon some cause outside of itself. Such an entity would explain the contingency of the universe’s existence. If one wants to deny this conclusion, then either the PSR must be rejected, or the universe must exist out of necessity; both alternatives seem improbable. What it is to be the cause of space-time needs further development, but to bring about the state of affairs constituting all of physical reality seems to be a uniquely powerful and metaphysical concept.
 Alexander R. Pruss and Joshua L. Rasmussen, “An Argument from Contingency,” in Necessary Existence (New York: Oxford University Press, 2018), 40.
 Peter J. Bussey, “God as First Cause – a Review of The Kalam Argument,” Science & Christian Belief 25, no. 1 (April 2013): 19-20, http://search.ebscohost.com/login.aspx?direct=true&db=aqh&AN=86934668&site=eds-live&scope=site.
 Michael Shermer, “The Grandest of Questions: Why Is There Something Rather Than Nothing,” Skeptic 23, no. 4 (2018): 13-15, http://search.ebscohost.com/login.aspx?direct=true&db=edsgao&AN=edsgcl.569823206&site=eds-live&scope=site.
 Bussey, 19-20.
 Bussey, 20.
 Alexander R. Pruss, “The Anti-theological Argument That There Are No Necessary Beings,” in The Principle of Sufficient Reason: A Reassessment (New York: Cambridge University Press, 2010), 94.
 Shermer, 8-9.
 Shermer, 8.
 Jacobus Erasmus and Anne Hendrik Verhoef, “The Kalam Cosmological Argument and the Infinite God Objection,” Sophia: International Journal of Philosophy and Traditions 54, no. 4 (2015): 412, doi:10.1007/s11841-015-0460-6.
 William Lane Craig, “Graham Oppy on The Kalām Cosmological Argument.” In The Kalām Cosmological Argument, vol. 1, ed. Paul Copan and William Lane Craig, (New York: Bloomsbury Academic, 2018), 159-60.
 Graham Oppy, “Craig and The Kalām Arguments.” In The Kalām Cosmological Argument, vol. 1, ed. Paul Copan and William Lane Craig, (New York: Bloomsbury Academic, 2018), 140.
 Oppy, 141.
 Alexander Vilenkin, “The Beginning of The Universe.” In The Kalām Cosmological Argument, vol. 2, ed. Paul Copan and William Lane Craig (New York: Bloomsbury Academic, 2018), 150-51.
 Vilenkin, 150.
 Vilenkin, 152.
 Vilenkin 152.
 Vilenkin, 152-53.
 Vilenkin, 153.
 Vilenkin, 153.
 Aaron C. Wall, “The Generalized Second Law Implies a Quantum Singularity Theorem.” In The Kalām Cosmological Argument, vol. 2, ed. Paul Copan and William Lane Craig, (New York: Bloomsbury Academic, 2018), 286-87.
 Vilenkin 153.
 Vilenkin, 153.