I received an interesting question from someone being presented with an objection from probability. The critic suggested that there is some positive probability that the universe should exist, and so in the infinite number of possible worlds that there are some number of universes that exist. We simply occupy one of those universes.
My response is below:
It sounds to me as though your friend may have become confused on what is meant by “possible worlds semantics”. When we use the language of possible worlds, we do not refer to other worlds that actually exist (such as in the multiverse hypothesis). Rather we are just using a semantic device to make it easy to discuss possibility. The only world that exists is the actual world (the actual world is the possible world which actually exists).
So, for example, we might say that there is a possible world in which the earth should have two moons instead of one. This is not to say that there are two worlds, one in which there is a single moon and another where there are two moons. Rather we are just saying that in the one world that does exist it could have been the case that earth should have two moons instead of one. That world, however, does not exist because as we can see there is only one moon. There is no sense in which the two-moon possible world exists. When we say, “There is a possible world in which the earth has two moons,” we don’t mean that this world exists but rather just that this was possible. A semantically equivalent way of saying that would be, “It is possible that the earth would have had two moons instead of one.”
So your friend’s appeal to chance here just doesn’t connect. In appealing to many worlds one is seeking to increase the probabilistic resources in order to overcome the improbability of an event. If I roll a six sided die, then there is a 1/6 chance that I should roll a 3. If I roll the die 6 times then there is a near 1/1 chance that I should roll a 3. Your friend’s mistake is in thinking that because there is a near infinite number of possible worlds that this increases the probabilistic resources for a universe’s existence in the actual world. There is only one world, one roll of the die if you will.
As you have pointed out, his principle of chance universes is ad hoc but it is also irrelevant. Unless there is some independent reason for thinking this principle is operative then it is ad hoc, simply invented as a proposed objection to the theistic arguments. More than that, however, it is irrelevant even if we concede it. The mere description of a principle or law does nothing to explain why that phenomena should take place. Consider the law of gravity. It certainly is the case that objects tend to fall towards each other, but my acknowledgement of that principle or law does nothing to explain away why objects tend to fall towards each other. Much scientific research has been done in this area and the theory of gravity has been developed as a result. The law of gravity doesn’t do anything to explain away the theory of gravity. The description that something tends to happen doesn’t explain away why it happens.
So if your friend wants to say that there is some probability that the universe should come into existence, we can just agree with him. Obviously there is some probability that the universe should exist because here we are. The probability that the universe should exist is surely not zero! Nonetheless, what we want to explore is why the universe exists (Leibniz) or why the universe began to exist (Kalam).
Now if he wants to say that there is some probability that the universe should begin to exist without a cause then he is simply denying premise 1 of the Kalam, and if he wants to say that there is some probability that the universe should exist without explanation then he is simply denying premise 1 of Leibniz’s argument. In that case we can just review the reasons for affirming these premises and ask him if he has any positive reasons for denying them.
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