Cosmos= The universe, all of existence. Cosmological arguments constitute a family of arguments for the existence of God.
So, what is a cosmological argument?
Throughout history, thinkers and reflective people have noticed that there is something fundamentally mysterious about reality. Something about reality itself calls out for some sort of extra-natural explanation. And it's not just certain aspects of reality- like the earth being habitable or living organisms being works of engineering- that call out for such an explanation, but every part of reality. Reality itself is mysterious. Philosophers throughout the ages have tried to put their finger on what is it about reality that makes it so mysterious, so explanation-worthy. Myriad answers have been offered- the phenomenon of motion, the phenomenon of change, the contingency or ephemeral nature of all objects, causality, and so on. Why is it that all things in the cosmos follow a strict, regular, universal order, as if guided towards some end?- asked the medieval theologian Thomas Aquinas, while Newton's contemporary Leibniz wondered about the contingency of all things (more on both soon). When you think about it, it is indeed mysterious that all of reality would be uniformly characterized by such features, where things could clearly have been otherwise. Things could have worked without the least whiff of regularity and lawlikeness, for example, or reality could have been a mosaic of necessary and contingent parts. These philosophers reasoned- the only way to account for these mysterious yet ubiquitous properties of the cosmos is to invoke the creative action of a being with properties similar to those of the God of Abrahamic theism, broadly conceived. That is the basic, intuitive thrust of a cosmological argument.
More precisely though, a cosmological argument uses as premise some large-scale feature of the cosmos- motion, change, order, contingency, causality, incidental properties, what have you- to argue for the existence of God.
This is a question that concerns the entire discipline of natural theology- why is it that philosophers always speak in terms of arguments and premises and conclusions and explanations and these debate-centric terminology? Why can't we just use our intuitions as guides to God? So in the case of the cosmological arguments for example- why don't we just appeal to the intrinsic mystery of these cosmic features as they are, instead of making rigorous, formulaic argument out of them?
The concern is valid. Indeed, sometimes the arguments of natural theology become so convoluted that we lose sight of its fundamental intuitive thrust. That's bad news. An evidence for God shouldn't only appeal to the logician in you, but also satisfy you at an intuitive, even emotional level. Understood this way, an argument is only as good as the level of faith it produces in you. And faith is as much a matter of the heart as it is of the intellect. That's why I'm a big proponent of exploring the intuition behind an argument before entering the more formulaic discussions concerning it. That's also why I find ontological arguments distasteful- not necessarily because they're weak, but because they are so hard to relate with and intuit.
That said, I'd say arguments are still important. You see, intuitions can only go so far as guides for truth. The human psyche is rife with potential logical fallacies and cognitive biases. It thus becomes difficult to tell a genuine intuition from an artificial, socially constructed one. Non-theists are cognizant of this fact, and for many of them the sole reliance of intuition as evidences for God may not just cut it. They may claim to not have those intuitions, saying there's nothing mysterious about the order in nature. Note that it's possible to 'talk yourself out' of even self-evident propositions. Especially in today's context where God is such an unpopular hypothesis, intuitions commonly associated with theism like order in nature and universal causality have largely fallen out of favor. Such a society is not conducive to theistic intuitions, and even if these intuitions are widely shared- people may explain them away as evolutionary spandrels.
That's where arguments come in. For someone who doesn't appear to share your intuitions, you need something more precise and objective. Arguments are what can demonstrate or prove the rational legitimacy of intuitions. This exercise is often taxing because you sometimes need to employ quite a set of terminology and analytic tools to lay out your intuitions in detail, and then defend them cogently. Trust me though, it turns out to be rewarding.
Arguments are even more required in the context of cosmology, since these arguments work with very little scientific data. They are primarily exercises in metaphysics, the branch of philosophy which deals with the ultimate truths of reality. The scope of metaphysics is so broad and unwieldy that there's little scope for scientific experimentation for confirming or disconfirming hypotheses. Sometimes such scopes do present themselves, however- physical cosmology is making it possible to test some metaphysical claims like whether the universe had a beginning. But this is the exception, not the rule. With so little data to work on, the only tool we have to sharpen our intuitions and 'quantify' them in some way is by employing structured arguments.
Examples of some cosmological arguments
As was alluded to above, many features of the cosmos are deemed to be mysterious by many, and used as fodder for cosmological arguments. Here are my three favorite examples. I'll just explain briefly their intuitive thrust here. The specific argument structures are for another day.
1. Contingency. Think about the chair you are (presumably) sitting in right now. Could it not have existed? Of course- the maker of the chair could simply have chosen not to make it, you could have chosen not to buy it, the human civilization could have evolved in such a way so that a chair were never invented- there are so many alternative possibilities. Same goes with the street in front of your house, or the building across it, or even you. Or even the whole neighborhood. Or even the earth. It seems that for all of these things, the possibility of them not existing is open. We can easily conjure up scenarios in the world's history which led to their non-existence. When you think about it, nothing in the world is such that it must have existed. Even laws of nature could have been different were the initial conditions of the cosmos different. Provided we live in a multiverse, perhaps there's another space-time continuum patch where gravity doesn't exist. All of the cosmos is characterized with this possibility of having been different than how they are now. This possibility of having been different is what philosophers call contingency.
Now clearly there are facts which are not contingent, but necessary. A circle could not for the life of it have possessed sides. A thing cannot but be identical to itself. The whole is necessarily greater than the part. These and many other facts are examples of facts which could not conceivably have been any different. A simple but imprecise way of characterizing necessity vs. contingency is thinking about them in terms of independence and non-self-dependence. Contingent facts seem to be dependent on something else for their obtaining, while necessary facts are true just because they are. They are true by virtue of their own nature, as they say. You can imagine necessity and contingency being two modes of being. Something can exist necessarily or it can exist contingently. A fact can be true, but it can be true in two modes- necessary or contingent.
When you think about the cosmos though, it seems to be characterized by just one mode of being- contingency. If everything were necessary, it wouldn't have been significant because necessary things are self-explanatory. There is nothing surprising about a circle having no sides, that's just the way it necessarily is. Reality could have also been a mosaic of necessary and contingent things. But the cosmos consists exclusively of things of one modality. Doesn't that call out for some external explanation? If reality consisted of human beings and nothing but human beings, it would be a perfectly legitimate question to ask- why are there human beings at all?- even if we had an explanation of each human being in terms of her parents. Similarly, since reality consists of contingent things and nothing but, it seems a plausible question to ask- why are there contingent things at all?
The theistic answer, of course, is that there is a Necessary Being- who exists not because of any external circumstance or explanation but because of its own nature- chose to bring about the existence of contingent things.
2. Causality. Instead of contingent vs. necessary, think of caused vs. uncaused things (the properties of 'being caused' and 'being contingent' are close, but not exactly identical). As with the previous example, our cosmos seems to consist exclusively of caused things. The property of causedness characterizes literally all of reality. Again, whence cometh caused objects? The answer closest at hand seems to be that an Uncaused being chose to bring about caused things in existence.
3. Order. Think of the coin that I'm holding in my hand. I have a firm conviction that if I were to drop it, it would fall to the ground. Where does this conviction emerge?
We human beings have an important and profound belief in the uniformity of natural laws. Not only is this belief necessary for functioning in everyday life, but it's crucial for the most advanced sorts of scientific research. I know if I jump from the rooftop, I will not randomly oscillate through the air but fall hard to the ground. Instead of turning into a green dragon, the sun would indeed faithfully rise in the East the next day. Sodium and chloride will continue to react to form table salt. Copper will continue to expand when heated. Justin Bieber will continue being uncool.
The reason we trust these intuitions is probably because these intuitions are a reflection of how things really are. There are actual solid laws of nature set in the fabric of the cosmos, and all things being equal, they wouldn't fail to obtain. If these regularities were just descriptions of nature as they are and not something deeper, as the Scottish philosopher David Hume famously believed, then our intuitions about the laws continuing to hold amounts to nothing. On Hume's view, the sun rising in the west has just as much probability as the sun rising in the east. One cannot escape the belief in laws, an universal order that governs all things.
Now how to explain this order? Scientific explanations clearly wouldn't suffice, since they assume the existence of laws beforehand. Science uses laws to explain things, although those things could be other laws. But to explain the existence of laws in general- that's where science would fall short, because there wouldn't be any law to explain the laws with. Some philosophical accounts of the law tried to explain this in materialistic terms, but I believe those explanations fail. The only way to explain them, again, is to posit a lawmaker and/or a law-upholder, an extra-natural being who preserves this order in nature. Et hoc decimus deum, as they say- all know this to be God.
Time for me to give some material on cosmological arguments.
1. First five podcasts from here
2. Paul Herrick, Job Opening: Creator of the Universe, here
3. Alexander Pruss and Richard Gale, A New Cosmological Argument, Religious Studies 35, 1999
4. Robert Koons, A New Look at Cosmological Arguments, American Philosophical Quarterly 34, 1997
5. Alexander Pruss, Principle of Sufficient Reason: A Reassessment
6. Alexander Pruss, The Leibnizian Cosmological Argument, in The Blackwell Companion to Natural Theology, eds. William Lane Craig and J. P. Moreland
7. Emanuel Rutten, Towards a Renewed Case of Theism: A Critical Assessment of the Contemporary Cosmological Arguments
8. John Foster, The Divine Lawmaker: Lectures on Induction, Laws of Nature, and the Existence of God.