Peter Unger

In this quite modestly ambitious essay, I'll generally just assume that, for the most part, our "scientifically informed" commonsense view of the world is true. Just as it is with such unthinking things as planets, plates and, I suppose, plants, too, so it also is with all earthly thinking beings, from people to pigs and pigeons; each occupies a region of space, however large or small, in which all are spatially related to each other. Or, at least, so it is with the bodies of these beings. And, even as each of these ordinary entities extends through some space, so, also, each endures through some time. In line with that, each ordinary entity is at least very largely, and is perhaps entirely, an enduring physical entity (which allows that many might have certain properties that aren't purely physical properties.) Further, each ordinary enduring entity is a physically complex entity: Not only is each composed of parts, but many of these parts, whether or not absolutely all of them, are themselves enduring physical entities, and many of them also are such physically complex continuing entities. When an ordinary entity undergoes a significant change, then, at least generally, this change will involve changes concerning that entity's constituting physical parts, whether it be a rearrangement of (some of) these parts, or a loss of parts, or a gain of parts, or whatever. Often, the entity will still exist even after the change occurs. As we may well suppose, this happens when, from two strokes of an ax, an ordinary log loses just a chip of wood. As we may then say, such a change conforms with the log's "persistence conditions." Somewhat less often, such an ordinary entity undergoes a change that means an end to it: When a bomb's explosion makes our log become just so many widely scattered motes of dust, the log will no longer exist. Such a momentous change doesn't conform with the log's persistence conditions.