LEGO Worlds, Change, and Causation
At this point, we have a sense of why it might be plausible that Humean supervenience is true, that is, that our world is fundamentally like a LEGO world. But many philosophers have found even that incredible, quite independently of the further question of how many worlds there are. Change, a pervasive and obvious phenomenon of our world, is absent in a LEGO world. Things change—cars move, babies grow, coffee goes cold, leaves turn brown. In contrast, a LEGO world is an utterly static world. (Remember that we are talking about creations from old-style LEGO sets, where no Krazy Glue is needed to keep things at rest.)
Lewis has a response on behalf of Humean supervenience: change, on his view, is variation along a fourth dimension. The world is fourdimensional, and we are four-dimensional space-time “worms” in it. Just as I have spatial parts—my head or my left arm, for example— I also have temporal parts. Change consists in the part located at one time having different properties from the part located at another time.
The idea that an extra dimension can, in a sense, account for change is familiar. In an old-fashioned slide show, images that are twodimensional (if we ignore their small depth) are stacked upon each other, and shown in sequence. If there are more than 16 shown per second, our eyes can no longer discriminate the individual images, and we see the slide show as a continuous film.
The sequential display of the slides is still a process that involves change: at one time one slide is being projected, at another time another. For this reason, an extra dimension can account for change “in a sense.” Change seems to be presupposed in the slide show case.
For this reason, Lewis and other four-dimensionalists need to say more to justify their claim that they can account for change.
Though it is debatable, let’s grant Lewis that our world is fourdimensional, and return to the question of how similar it is to a LEGO world. Since a LEGO world is three-dimensional, we have identified another important respect in which they differ. But this is arguably a relatively superficial difference, and does not detract much from the fundamental similarity of LEGO worlds and the real world. A proponent of Humean supervenience can think of the real world as being like a sequence of LEGO creations, stacked upon each other in a fourth dimension. This would account for the fact that things change.
But according to critics of Humean supervenience, this way of thinking about the world still leaves out one of its crucial features: that some things cause others, and correspondingly, that some things are the effects of others. If I step on your toe, and you subsequently feel pain, there are not just two things happening in succession. Rather, my stepping brought the pain about—or so it seems. But even if LEGO creations are stacked upon each other in a fourth dimension, what is the case in one of them does not in any way bring about what is the case in the next one. They are, in that sense, all independent from each other.
This issue marks a fundamental divide in contemporary metaphysics between Humeans and non-Humeans. Hume denied that there are necessary connections, by which he meant causation. Watching football, I observe feet approaching and then touching a ball, and the ball subsequently moving away. I do not observe any mysterious connection between the two things—I do not observe the one thing causing the other. I think that there is causation merely because I observe kicking regularly being followed by ball movement.
Lewis does not deny outright that there is causation in the world. But like Hume, he does not take it to be one of its fundamental features. His idea is that causation is a matter of what happens in similar worlds. Here the other possible worlds that he controversially believes in turn out to be useful for him. My stepping on your toes causes you pain because in those worlds in which I do not step on your toes, and that are otherwise maximally similar to our world, you are not in pain. The idea is that a cause is something that makes a difference, with respect to the effect, between our world and a similar world.
Whether Lewis turned this idea into a successful theory of causation is, again, a large and contested issue. If he is successful, then that also vindicates the idea that our world is fundamentally like a LEGO world. We can then use our judgments about which LEGO worlds are similar to which other ones to make claims about what causes what in a four-dimensional LEGO world.
In his ingenious defense of Humean supervenience, Lewis has shown that LEGO worlds might be very much like the actual world, and thus when we create a LEGO world, we might be making a toy model of the actual world that captures its metaphysically significant features.