Our talk of external things, our very notion of things, is just a conceptual apparatus that helps us to foresee and control the triggering of our sensory receptors in the light of previous triggering of our sensory receptors. The triggering, first and last, is all we have to go on. In saying this I too am talking of external things, namely, people and their nerve endings. Thus what I am saying applies in particular to what I am saying, and is not meant as skeptical. There is nothing we can be more confident of than external things — some of them, anyway — other people, sticks, stones. But there remains the fact — a fact of science itself — that science is a conceptual bridge of our own making, linking sensory stimulation to sensory stimulation; there is no extrasensory perception.