Why Does the World Exist: An Existentia - By Jim Holt Page 0,39

concurrent in time. Moreover, not all explanations must invoke causes. Think, for example, of the explanation for a rule in baseball or a move in chess.

Hume’s second argument is a better one. Suppose (he has his spokesman Cleanthes say) we think of the history of the world as a series of events. If the world is eternal, this series is an infinite one, with no first or last member. Now, each event in the series can be causally explained by the event that precedes it. Since there is no event that lacks an explanation, everything seems to be explained. “Where then is the difficulty?” Cleanthes asks. He is unimpressed by the obvious rejoinder: that even if each event in the series is causally explained in terms of an earlier event, the series as a whole remains unexplained. For the series as a whole, he insists, is not something over and above the events of which it is composed. “I answer that the uniting of these parts into a whole, like the uniting of several distinct countries into one kingdom, or several distinct members into one body, is performed merely by an arbitrary act of the mind, and has no influence on the nature of things,” Cleanthes says. Once all the parts are explained, he submits, it’s unreasonable to demand a further explanation of the whole.

Seen in this light, an eternal world looks like the cause of itself, since everything within it is caused by something else within it. Hence it requires no external cause for its existence. It is causa sui—an attribute usually reserved for God.

But there’s still something missing here. This infinite world is like a railroad train with an infinite number of carriages, each pulling the one behind it—and no locomotive. It can also be likened to a vertical chain with an infinite number of links. Each of these links holds up the link below it. But what holds up the chain as a whole?

Imagine yet another sort of series that has no beginning and no end, this one consisting of an infinite succession of copies of some book—say, the Bhagavad Gita. Suppose that each book in the series is faithfully copied by a scribe, letter for letter, from the preceding book in the series. Now, for each given copy of the Bhagavad Gita, the text is fully explained by the text of the preceding copy, from which it had been transcribed. But why should the whole series of books, extending back infinitely far in time, be copies of the Bhagavad Gita? Why not copies of some other book—Don Quixote, say, or Paradise Lost? Why, for that matter, should there be any books at all?

The preceding thought experiment, essentially due to Leibniz, is somewhat fanciful. But it can be sharpened up and made more scientific. Suppose you want to explain why the universe is the way it is at a given moment in its history. If the universe is eternal, you can always find earlier states in its history that are causally related to the state you’re trying to explain. But knowledge of those earlier states is not enough. You must also know the laws that govern how one state of the universe evolves into another.

To be more precise, consider the total mass-energy of the universe as it is today. Call this mass-energy M. Why does M happen to have the value it does? To answer that question, you might point out that total mass-energy of the universe yesterday was also M. But that is not by itself an explanation of its value today. You also need to appeal to a law—in this case the law of mass-energy conservation. The total mass-energy of the universe today is M because (1) the total mass-energy of the universe yesterday was M and (2) mass-energy is neither created nor destroyed. Now your explanation is complete.

Or is it? It appears that there are two ways in which the universe could have been radically different. It could have had a different total mass-energy throughout its history—say, M' instead of M. And it could have had a different law governing that mass-energy: a law that, for example, might have allowed the mass-energy to cycle back and forth over time between a pair of values, M and Mˇ. (To return to the Bhagavad Gita example for a moment, that would be as if the text kept getting translated back and forth from Sanskrit to English to Sanskrit to English and so on.) We still have