On examining, accordingly, what the mind does when it applies the principle of the Composition of Forces, I found that it performs a simple act of addition.

It adds the separate effect of the one force to the separate effect of the other, and puts down the sum of these separate effects as the joint effect.

But is this a legitimate process?
In dynamics, and in all the mathematical branches of physics, it is; but in some other cases, as in chemistry, it is not; and I then recollected that something not unlike this was pointed out as one of the distinctions between chemical and mechanical phenomena, in the introduction to that favourite of my boyhood, Thompson's _System of Chemistry_.

This distinction at once made my mind clear as to what was perplexing me in respect to the philosophy of politics.

I now saw, that a science is either deductive or experimental, according as, in the province it deals with, the effects of causes when conjoined, are or are not the sums of the effects which the same causes produce when separate.
It followed that politics must be a deductive science.