Squeeze your fingers around something hard. It feels like you’re making an effort with your fingers. But you’re making an effort with muscles that are in your forearm rather than in your fingers—fingers have no muscles inside them.
Fingers, for instance, don’t make it in. A typical finger has three segments, but the muscles to move these segments are, as we saw, far away from the finger. What is included in the finger, assuming it’s a real object? Presumably the tendons that move the segments had better be included. But these tendons extend through the wrist to the muscles. Looking at anatomical pictures online, they are continuous: they don’t have any special boundary at the base of the finger. Moreover, blood vessels would seem to have to form a part of the finger, but they too do not start at the base of the finger.
Perhaps the individual bones of the finger are naturally delineated parts? But bones only have delineated boundaries when dead. For instance, living bones have a nutrient artery and vein going into them, and again based on what I can see online (I know shockingly little about anatomy—until less than a year ago, I didn’t even know that fingers have no muscles in them), it doesn’t look like there is any special break-off point where the vessels enter the bone.
Perhaps there are some things that have delineated boundaries. Maybe cells do. Maybe the whole interconnected circularity system does. Maybe elementary particles qualify, too. But once we see that what are intuitively the paradigmatic parts of the body—things like fingers—are not in the ontology, we gain very little benefit vis-à-vis common sense by insisting that we do have proper parts, but they are things that require science to find. It seems better—because simpler—to say that in the correct ontology the body is a continuous simple thing with distributional properties (“pink-here and white-there”). We can then talk of the body’s systems: the circulatory system, the neural system, ten finger systems, etc. But these systems are not material parts. We can’t say where they begin an end. Rather they abstractions from the body’s modes of proper function: circulating, nerve-signaling, digital manipulating. We can talk about the rough locations of the systems by talking of where the properties that are central to the explanation of the system’s distinctive functioning lie.