We are interested in this chapter in a number of questions about physical (or material) objects. That’s one main reason to want to be able to distin- guish between physical and non-physical objects. Another main reason has to do with the role this distinction plays in many areas of philosophy,
including The Mind–Body Problem and discussions of various forms of materialism.1 So what exactly are physical objects?
George Berkeley said (roughly) that physical objects are the things out- side of our minds that can be sensed.2 (He also ended up arguing that
there can be no such things, but his arguments for that conclusion can be divorced from his account of physical objects.)
one difficulty with Berkeley’s sensational account of physical objects is that different beings have different sensory abilities, which means that this account threatens to make the notion of a physical object a relativis- tic one. That would be a highly undesirable result, given the crucial role played by the concept in various philosophical debates. In order to avoid this result, then, we could modify the sensational account to say that physical objects are objects that can be sensed by some sentient being or other. But if we say this then we run the risk of casting our net much too wide; for it may be possible for a disembodied mind to sense another such mind, and it may also be possible for an extremely sensitive creature to sense such seemingly non-physical entities as propositions. And there is also this objection to the sensational account: it turns the property of being a physical object into an extrinsic property of the things that have it; and yet, intuitively, being physical is an intrinsic property of physical objects.
An alternative account of the physical holds that physical objects are the objects studied by physics.3 This appears to be a popular approach,
especially among philosophers writing on physicalism. But it leads very quickly to a definitional circle if, as we suspect, the best definition of ‘physics’ is the study of physical objects.
In addition, there are a number of apparent counterexamples to the physical theory account of physical objects: numbers, equations, formulas, functions, properties, and propositions all seem to be among the objects studied by physics, but none of them strikes us as a physical object. Also, it is quite possible that some day it will turn out that the best physical the- ory is one that (correctly, we can suppose) posits ghosts and goddesses and other entities that we would not want to call physical objects.
1Chapter 6 discusses several versions of materialism. our focus in that chapter was the nature of mental states.
2 See Berkeley, A Treatise Concerning the Principles of Human Knowledge, Part I, section 4. 3 See, for example, Smart, Our Place in the Universe, p. 79.
W. V. o. Quine has suggested that a physical object is “the aggre- gate material content of any portion of space-time, however ragged and discontinuous.”4 on the face of it, this proposal looks circular, since we
are taking ‘material’ to be synonymous with ‘physical’. But it may be pos- sible to avoid this problem simply by subtracting the word ‘material’ from Quine’s suggestion, leaving us with a proposal according to which a phys- ical object is the content of any portion of space-time.
This proposal has some plausible consequences, but it also comes with some noteworthy metaphysical baggage. For this Quinean account entails a thesis called Universalism (see below), according to which (roughly) for any group of objects, there is a further object whose parts are the mem- bers of that group. Thus, for example, according to Universalism, there is an object whose parts are your head, the president’s shoes, and a single quark from the surface of the moon. As we will see in 8.8 below, there are some interesting arguments in support of Universalism; but it is neverthe- less a highly controversial thesis. Thus, the fact that our Quinean account of physical objects entails Universalism counts as a significant cost of that account.
A further problem for the Quinean view is that it may prove impossible, on that view, to distinguish between a particular physical object, such as a ball, say, and any event that is spatio-temporally coincident with that ball, such as the ball’s history. What’s more, on certain theories of universals, it may also prove impossible to distinguish between the ball and its various life-long properties. (Perhaps this problem is the reason Quine included the word ‘material’ in his statement of the view in the first place.)
Peter van Inwagen has suggested an alternative account of physical objects.5 According to van Inwagen, there is a certain family of proper-
ties – such as being located in space, having spatial extension, persisting through time, being able to move about in space, having a surface, hav- ing mass, being made of matter, and so on – that are associated with the concept of a physical object. Van Inwagen further suggests that the latter concept is an imprecise one, and that the extent to which an object exem- plifies all or most of the concepts on the associated list is the extent to which that object is a physical object.
4 Quine, “Whither Physical objects?,” p. 497. 5 van Inwagen, Material Beings, p. 17.
This common-sense account of physical objects may well be an adequate way of capturing the everyday notion of a physical object. But when it comes to the concept of a physical object that is featured in the disputes of philosophers, the common-sense account is problematic. one difficulty is that it makes the concept of a physical object a vague one, which is undesirable given the role that concept plays in numerous philosophical disputes.
Another problem for the common-sense account is that it entails that quarks, electrons, atoms, and even certain large organic molecules may not be physical objects.6 This is bad for two reasons. First, since quarks,
electrons, and so forth are all among the parts of macroscopic physical objects, this consequence of the common-sense account goes against the very plausible thesis that a physical object cannot have a non-physical object among its parts. And second, no one on either side of the debate over physicalism thinks that the existence of quarks and electrons refutes physicalism.
There may be another problem facing the common-sense account of physical objects. Imagine a possible world with very different properties and laws of nature from those in the actual world. Suppose that in this other possible world, there are point-sized objects with spatial locations that never persist through time but, instead, pop in and out of existence instantaneously. Since they don’t persist, these objects of course don’t move around. Suppose further that they don’t have mass, or any of the other familiar properties of physical objects in this world, but instead exemplify various alien properties that nothing in the actual world ever exhibits; and suppose finally that it is these strange, alien properties that figure in the laws of nature governing this other possible world. The pro- ponent of the common-sense account has to say that there are no physical objects in such a world (since the objects in question have almost none of the properties on the relevant list); but this seems like a strange result. For
6 For as van Inwagen himself says, “one has to be very careful in ascribing any of the features in the above list to such things; and talk about the surfaces of submicro- scopic objects, or about the stuffs they are made of, tends to verge on nonsense” (Material Beings, p. 17). It should be noted, however, that van Inwagen considers it a feature of his view, rather than a bug, that it seems to entail that quarks, etc., are not material objects.