Substance is thought to belong most obviously to bodies; and so we
say that not only animals and plants and their parts are substances,
but also natural bodies such as fire and water and earth and
everything of the sort, and all things that are either parts of
these or composed of these (either of parts or of the whole bodies),
e.g. the physical universe and its parts, stars and moon and sun.
But whether these alone are substances, or there are also others, or
only some of these, or others as well, or none of these but only
some other things, are substances, must be considered. Some think
the limits of body, i.e. surface, line, point, and unit, are
substances, and more so than body or the solid.

    Further, some do not think there is anything substantial besides
sensible things, but others think there are eternal substances which
are more in number and more real; e.g. Plato posited two kinds of
substance-the Forms and objects of mathematics-as well as a third
kind, viz. the substance of sensible bodies. And Speusippus made still
more kinds of substance, beginning with the One, and assuming
principles for each kind of substance, one for numbers, another for
spatial magnitudes, and then another for the soul; and by going on
in this way he multiplies the kinds of substance. And some say Forms
and numbers have the same nature, and the other things come after
them-lines and planes-until we come to the substance of the material
universe and to sensible bodies.

    Regarding these matters, then, we must inquire which of the common
statements are right and which are not right, and what substances
there are, and whether there are or are not any besides sensible
substances, and how sensible substances exist, and whether there is
a substance capable of separate existence (and if so why and how) or
no such substance, apart from sensible substances; and we must first
sketch the nature of substance.