Let us suppose that I am playing a nice game of Xiangqi with my daughter. The pieces are wooden, with painted red or black characters. As we play, we notice that one of the red catapult pieces has the character on its surface half warn off. So, I grab a laser pointer, and some motors, and a little processor, and program the motors to move the laser pointer rapidly so as to trace out the missing half of the character drawn on the piece, indeed to do so so rapidly that it looks like the character is all in place without any flicker. Moreover, I add a little camera that tracks the wooden disc, and continually repoints the whole assembly to follow the disc, so that when I move the disc, the character drawn on it moves with it. As we keep on playing (maybe the paint on the piece is of very low quality), more and more of the character wears off, and I am continually reprogramming the motors to replace more and more of the paint by the laser beam. Eventually, the disc has no paint on it, but has a laser image of the catapult character on it. I then take a bite out of the wooden disc itself. (Maybe I'm a beaver.) And I keep on taking more bites, while reprogramming the laser beam not to track what's left of the wooden disc, but to track my finger motions. Finally, I finish eating the wooden disc. I can now finish the game of Xiangqi with my daughter, except that now one of the red catapult pieces has become a pattern of laser light.
Suppose we were tempted to say that the initial red catapult piece was a substance.[note 1] Now, surely, a pattern of laser light isn't a substance. Light-spots aren't even processes, but quasi-processes. They can, after all, "move" faster than light.[note 2] So then we have the oddity that a substance became a non-substance. But it is plausible, for the same reasons for which it is plausible to suppose that the Ship of Theseus survives the replacement of its parts, that the red catapult piece survives its replacement with a light spot. Thus, if the red catapult piece initially was a substance, then one and the same thing is an existent substance at t0 and an existent non-substance at t1. Moreover, this didn't take any kind of miracle—all it took was a bit of skill with motors and electronics. If this seems absurd to you, then you may wish to conclude that the initial red catapult piece was not a substance.
Similarly, a painting could be restored with new paint, or it could be repaired piece by piece by projected light. A chair could be replaced, piece by piece, with force-fields, perhaps, or maybe even with carefully aimed jets of air. A computer could be replaced, piece by piece, with an emulation of itself (just replace the parts one-by-one with emulations). If these things survive this kind of replacement, and if we do not wish to accept that there can be change between categories—that a substance can change into a non-substance, say—at least without a miracle[note 3], then the conclusion to draw is that Xiangqi pieces (and by the same token international chess pieces), paintings, chairs and computers are not substances. In fact, it seems plausible to generalize this: perhaps no artefact is a substance, unless perhaps it is wholly composed of one non-artefactual substance (such as Peter van Inwagen's example of the hammock that is made out of one snake, weaved into a hammock like a rope).
Some of us want to go one step further, and deny that artefacts, except perhaps if wholly composed of one non-artefactual entity, exist. For it is not implausible to say that spots of light don't exist, and anything that can change into a spot of light doesn't exist either.