 One question to ask about the category of object is to ask what falls under it—what is in its extension? This is the Extension Question. The extension of some categories is obvious. The extension of the category electron is all the electrons and no non-electrons. The extension of the category number is all and only the numbers. The extension of the category composite things is all and only the things that have proper  parts.[12]  For some categories, difficult questions arise. Some of those questions involve the question of whether certain things are in the extension of the category, e.g., is the Pope in the extension of the category bachelor? Is a fetus in the extension of the category person? Some questions concern the nature of the things falling under the category. The extension of the category book includes all and only the books. Does it include ebooks? Does it include some particular and well-loved copy of Material Beings, with its stained pages and cover, or the abstract type of which this particular copy is a token? Answering the Extension Question leaves many other questions unsettled, one of which is the nature of the things that are in the extension. But knowing which things are in the extension is a good start toward figuring out their nature.