Perception is a species of a wider genus, namely _sensitivity_. Sensitivity is not confined to living things; in fact it is best exemplified by scientific instruments. A material object is said to be “sensitive” to such and such a stimulus, if, when the stimulus is present, it behaves in a way noticeably different from that in which it behaves in the absence of the stimulus. A photographic plate is sensitive to light, a barometer is sensitive to pressure, a thermometer to temperature, a galvanometer to electric current, and so on. In all these cases, we might say, in a certain metaphorical sense, that an instrument “perceives” the stimulus to which it is sensitive. We do not in fact say so; we feel that perception involves something more than we find in scientific instruments. What is this something more? The traditional answer would be: consciousness. But this answer, right or wrong, is not what we are seeking at the moment, because we are considering the percipient as he appears to an outside observer, to whom his “consciousness” is only an inference. Is there anything in perception as viewed from without that distinguishes it from the sensitivity of a scientific instrument? There is, of course, the fact that human beings are sensitive to a greater variety of stimuli than any instrument. Each separate sense-organ can be surpassed by something made artificially sensitive to its particular stimulus. Photographic plates can photograph stars that we cannot see; clinical thermometers register differences of temperature that we cannot feel; and so on. But there is no way of combining a microscope, a microphone, a thermometer, a galvanometer, and so on, into a single organism which will react in an integral manner to the combination of all the different stimuli that affect its different “sense-organs”. This, however, is perhaps only a proof that our mechanical skill is not so great as it may in time become. It is certainly not enough to define the difference between a dead instrument and a living body.