206 Proceedings of the Boyal Society of Edinburgh. [Sess. 
would, judged by the light of common sense, be an absurdity. But a 
principle which common sense brands with absurdity is philosophically 
false and a scientific blunder.” 
The following considerations may be regarded as the outcome of his 
philosophical studies at the university : — “ The sure and invincible method 
of arriving at every truth which the mind is capable of discovering is 
always one and the same. It is that, namely, of throwing away all 
generalization, of going back to first principles, and of letting the mind 
be guided by these alone. ... To reach the structure of words we must 
go back ... to the letters. . . . In mathematics the starting-point is not 
generalizations, but axioms. ... In metaphysics Descartes led the way of 
progress by analysing till he thought he could reach some ultimate 
elements beyond which it was impossible for him to go, then studying 
their force and power, and proceeding synthetically. . . . On the other 
hand, . . . Gerhardt’s generalization . . . leads him ... to restrict 
chemical science to the arrangement of bodies according to their decom- 
position, and to deny the possibility of our comprehending their molecular 
constitution.” 
Couper examines less fully the radical theory and the theory of 
copulse. The radical theory ascribes to the radicals the character 
of elements, of ultimate powers, and thus stops at the very point 
where an explanation is required. The theory of bodies conjugated 
by addition divides chemical compounds, if possible, into two 
parts, but gives no account of the force which holds these parts 
together. 
We cannot but see that it is, for Couper, a philosophical necessity, in 
-seeking an explanation of the molecular structure of chemical compounds, 
to go back to the chemical elements of which they are composed, and in 
the very first place to find out the properties and functions of these 
elements. The whole of chemistry must, he says, be considered as one. 
The general principles common to all the elements and those peculiar to 
each must be determined. In each known chemical compound the pro- 
perties and relations of each element contained in it must be taken into 
consideration. 
After this philosophical introduction Couper developes his theory, 
establishing individual points more fully here than in the short com- 
munication in the Comptes renclus. Nowhere does the contrast in the 
mode of thought between Couper and Kekule appear more clearly 
than in the philosophical foundation which Couper gives to his theory, 
rejecting Gerhardt’s type theory, the radical theory, and the theory 
