242 Proceedings of the Royal Society of Edinburgh. [Sess. 
combination diminishes somewhat the value of the otherwise great logical merit of 
this system. 
Having taken notice of such exceptions, the empirical truth of the theory may 
be otherwise admitted. 
The philosophical test demands that a theory be competent to explain the 
greatest number of facts in the simplest possible manner. 
In applying this test, three aspects of it require to be taken into consideration : — 
1. As to the extension of the theory. 
2. The explanation it affords of the facts. 
3. The manner of this explanation. 
As to the first : this theory indeed brings every chemical combinate under a 
certain comparative point of view with every other. Herein apparently is its merit. 
Nevertheless, should our test be applied to its full extent, it will be found that it 
is fatal to this system, in other respects so imposing. The comparative point of view 
which it adopts is fundamentally false. 
As to the second: it does not explain the facts at all; consequently the most 
essential point of the test is unfulfilled. 
3. This condition of the test is in like manner unfulfilled from the fact of the 
second not being complied with. 
Why is it that Gerhardt’s theory so signally fails in these two essential requisites 
Because it is based upon an old but vicious principle, which has already retarded 
science for centuries. It begins with a generalization, and from this generalization 
deduces all the particular instances. But it does not come within the limits of a 
chemical paper to enter upon a discussion which is purely metaphysical. Nevertheless, 
the theory of Gerhardt can only be combated upon metaphysical grounds, because it 
is only in overturning a general principle of research that the theory can be proposed. 
Gerhardt’s generalization lacks, moreover, the merit of being represented by a type 
having a known existence. 
from which he derives every chemical combinate, 
being in itself indefinite, cannot of course be contained or be produced in any definite 
body. That, however, which may be demanded of the type is, that in itself it 
should afford at least an instance of that which it is meant to represent. Now the 
. TJ 
part “ n” of the type represents the notion of indefinite multiples of 0 . But not 
H 
H 
a single instance of a multiple of 0 has been proved to exist ; much less has it 
H 
been proved that there exists, or can exist, multiples of this body in an indefinite 
series. The perfection or imperfection of the type meant to represent the generalized 
notion is, however, a matter of comparatively inferior moment. It is the principle 
involved in this generalization which is essentially pernicious. 
Should the principle which is therein adopted be applied to the common events 
of life, it will be found that it is simply absurd. Suppose that some one were to 
