THEORY OF MATTER. 41 



speak, less that is discontinuous, than any which should recog- 

 nise other principles, for instance chemical affinity, distinct from 

 force without enquiring into the relation which subsists between 

 them. 



5. It may however be asserted that this enquiry is alto- 

 gether superfluous that the power of exerting attractive or 

 repulsive force is one property of matter, that chemical affinity 

 (and so in other cases) is another that the two are not merely 

 distinct, but absolutely independent and heterogeneous. But to 

 this view the arguments which seem to have led to the adoption 

 of a purely mechanical system, appear to prevent our assenting. 

 I shall therefore attempt to state what I conceive these argu- 

 ments to have been. 



6. It is a fundamental principle of the secondary mechanical 

 sciences, for instance of the theory of light, that the secondary 

 qualities of bodies are to be explained by means of the primary. 

 Every substance, to use for a moment the language of Leibnitz, 

 is essentially active ; in other words it is to be conceived of as 

 the formal cause of the sensible qualities which are referred to it. 

 If we ask why gold is yellow and silver white, the answer at 

 once presents itself that the diiference of colour corresponds and 

 is due to a difference between the essential constitution of the 

 two substances. Now the essential constitution here spoken of, 

 and consequently the differences which individuate it in different 

 cases, may conceivably be something altogether incognisable to 

 the human intellect. The notion that it is so was expressed 

 scholastically by saying that substantial forms are not cog- 

 noscible. But if, setting aside this opinion, we affirm that the 

 essential constitution of each substance is a matter of which the 

 mind can take cognisance, we are led at once to the distinction 

 between primary and secondary qualities. The first are ascribed 

 to each substance as its essential attributes, in virtue of which 

 it is that which it is the second result from the primary*, (by 

 which as we have said the essential or formal constitution of the 

 substance in question is determined,) and have reference to the 

 mind by which they are perceived, while the primary are 

 ascribed to it independently of any reference to a percipient 



* Or that which in its formation it was to be, rb ri ty cli>at. 



