Chemistry Reference
In-Depth Information
Chapter 7
One Substance or More?
Paul Needham
7.1
Introduction
The ancients disputed whether matter comprises a single or several substances, but
by Aristotle
s time several different substances were distinguished. Matter was then
understood to be neither all of the same kind nor of continuously varying kinds, but
as comprising particular substances that can be recognised as the same substance
when we come across them on different occasions and in different circumstances.
The identification of a particular substance among many therefore depends upon
criteria of being the same substance. But picking out a particular substance in the
first instance proceeds, we might think, without prior formulation of any such
criteria, relying instead on a general criterion of being a single substance. Once
we have our sample, we can then proceed to find characteristic features in terms of
which criteria of being the same substance can be formulated.
Epistemologically, there may be something to the priority of the notion of being
a single substance over the general relation of being the same substance. But the
former is defined in terms of the latter, which is the logically prior notion. This is
apparent from the approach of Aristotle, who took homogeneity to be the criterion
of being a single substance. A quantity thus delimited by natural phase boundaries
provides an observable sample which can then be studied for characteristic features.
Portions of this can be investigated for properties, which might involve transfor-
mation into different substances, without loss of the entire original sample. The use
of portions in this way is sanctioned by Aristotle
'
s criterion of being a single
substance since he maintained that all substances (elements and compounds)
are homogeneous, or as he puts it, homoeomerous—comprise like parts:
“if combination has taken place, the compound
must
be uniform—any part of
'
