 A decent test for distinguishing between extrinsic and intrinsic properties is this. A property F is an extrinsic property of an object o if o having F implies that something distinct from, and not a proper part of, o exists. For example, being married to Sally is an extrinsic property of John's since John can only have that property if something, Sally, distinct from, and not a proper part, of him exists. A property F is an intrinsic property of o if and only if o having F is compatible with nothing apart from o and its proper parts existing, and also compatible with something apart from o existing. For example, the property of being round is an intrinsic property because a surface S having that property is compatible with nothing other than S and its proper parts existing. Here is one reason why this is only a rough way of drawing the distinction between intrinsic and extrinsic properties. It looks as though the property of existing in complete isolation, which Lewis calls loneliness, is extrinsic. Nevertheless being, in this sense, lonely is not only compatible with, but requires that nothing else exists. (For more discussion, see the entry on  intrinsic vs. extrinsic properties.)  