4. Why a privative cause in nature may occasion a positive idea.
If it were the design of my present undertaking to
inquire into the natural causes and manner of perception, I should offer this as a reason why a privative cause
might, in some cases at least, produce a positive idea; viz., that all sensation being produced in us only by
different degrees and modes of motion in our animal spirits, variously agitated by external objects, the abatement
of any former motion must as necessarily produce a new sensation as the variation or increase of it; and so
introduce a new idea, which depends only on a different motion of the animal spirits in that organ.