My occasional metaphysical sparring partner, Ian Spencer, poses the following objection
to presentism - the view that only what exists now exists simpliciter:
Here's a problem for most forms of presentism - on most versions, there's no good answer to the question of why this time is the present one. That is, why does the stuff at this time (and in its current arrangement) exist rather than that of some other? Why isn't some other time the present one?
What I find puzzling here is the fact that Ian thinks this is a problem for presentism
. After all, for the presentist the present time is the only time there is - the past is no more and the future is not yet. So to ask "why is this time the present?" sounds to a presentist's ears like asking "why is the present the present?" Not a very interesting question.
What I think Ian has in mind is a somewhat different question: "Why does the present have the configuration it now does?" The obvious answer is to say that the present configuration of the world is as it is because it developed from an earlier configuration in accordance with the causal powers, dispositions, choices, etc. that were in effect at that earlier state.
Here's an analogy. Imagine a world with the same physical laws as ours in which there is only a single homogeneous lump of radioactive material that God has just created ex nihilo
. According to physics, the atoms composing this lump have a certain disposition to undergo radioactive decay. The strength of this disposition is reflected in the radioactive half-life of the substance. The lump will also have a disposition to emit photons due to a phenomenon known as black-body radiation. Finally, the atoms of the substance and the electrons on those atoms have kinetic energy, which depends on velocity, which is an instantaneous directional tendency to be elsewhere. Now, given the inherent dynamic tendencies in the lump, it will not, indeed cannot, remain in its initial state. But the moment something, anything happens -
decay, photon emission, electron motion, etc. we no longer have the same
configuration. The radioactive lump system, due to its own internal tendencies and causal powers, has changed from one configuration to another. And with change comes time. At each moment in the evolution of the system it is in a different state. Each state is the consequence of previous states. And, unlike what B-theorists seem to think, there is no need for previous states to "stick around" as static parts of a tenseless block universe.
In my experience, I've found that many people, philosophers included, are held captive by a certain conceptual metaphor when it comes to thinking about time. The metaphor is that of the "timeline". According to this metaphor, time is "stretched out" along a dimension and consists of multiple events ordered by temporal relations like "earlier than" and "later than". It's a familiar picture from history books. But it's a picture that presentists (with the partial exception of ersatzist presentists like Bourne and Crisp) thoroughly repudiate. For presentists, time is not a line. It does not consist in "earlier than" relations between events. It consists, rather, in the intrinsic dynamism of reality, especially the tendencies of things to become other than they now are. Ian may find that picture "spooky". So be it. I'm inclined to say the same about his block universe. We may just have to agree to disagree on this issue.