For it is easy enough to explain all the different characters encountered among the minds of men, on the one hand by the abundance and scarcity, by the rapidity or slowness of agitation, and by the density or lightness of the animal spirits, and on the other hand by the delicacy or coarseness, the moistness and dryness, and the malleability of the brain fibers; and finally, by the relation the animal spirits might have to these fibers.