Chemistry Reference
In-Depth Information
such a compound is the same as the whole, just as any part of water is water” (
DG
I.10, 328
a
10f.). Here we see how the notion of sameness of substance comes in, a
quantity of matter being a single substance if all its parts are of the same substance.
As understood above, portions of a sample are spatial parts (exhausting
the matter occupying a region which is a proper part of the region occupied by
the sample). Since Aristotle held that cooccupancy was impossible, the parts at
issue in the passage just quoted were therefore considered to be spatial parts. After
Aristotle, the Stoics thought of blends (at this time there was no generally
recognised distinction between what we would call compounds and solutions) as
comprising different elemental substances occupying the same place at the same
time. Although many have followed Aristotle in denying the possibility of
cooccupancy (albeit still unable to improve on Aristotle by offering an independent
supporting argument), there is by no means a unanimous consensus on the issue.
1
If the possibility of cooccupancy is recognised, then the definition of being a single
substance must be formulated with a restriction to spatial parts along the lines of: a
quantity of matter is a single substance if all its spatial parts are of the same
substance.
2
Identifying a single substance involves identifying a quantity all of
whose spatial parts are the same substance.
Aristotle
s criterion of being a single substance has long been abandoned in
science, which for very good reasons distinguishes between substance and phase,
allowing that a homogeneous quantity might contain several substances (e.g. a solu-
tion such as sea water or the air) and that a heterogeneous quantity might comprise a
single substance (such as a mixture of ice and liquid water). Whether a quantity
contains a single substance is not in general directly discernible by observation, even if
the quantity of matter is. Chemistry has become more of a theoretical science as it has
developed over the course of history and we would expect this to be reflected in its
notion of substance, which has developed accordingly without preserving all its
historical trappings. Whether different theoretical sources pull in the same direction,
and in particular whether macroscopic and more recent microscopic perspectives are
accommodated in a unified general picture, is not obvious, suggesting that the notion
of a substance is not as straightforward as it might at first appear.
'
7.2 Thermodynamic Criteria of Sameness and Distinctness
A criterion of comprising a single substance should be general rather than varying
from one substance to another, preferably based on an appropriate general theory.
Aristotle
s homogeneity criterion was general, but although part of an impressive
theoretical stance in its time, cannot now be considered so. The notion of substance
'
1
See Duhem (
1892
, pp. 271-273; 1893, p. 304; 1894, pp. 240-1), Tisza (
1977
), p. 128 and further
remarks in this spirit in Needham (
2007
), pp. 41-2.
2
For an explicit formulation of this and related principles governing the same substance relation,
see Needham (
ms
).
