It is essential, at the very threshold of logic, to distinguish between a percept, which is what the senses perceive, and which is an object of study for the intellect, and a perceptual fact, which the understanding perceives in the percept, and which is the first fruit of observation. Photographs, even when they show no more than the eye can see, are most valuable in sciences of observation; they are stored up percepts. But they in no degree enable us to dispose with scientific descriptions, which are records of perceptual facts, the basis of all else in science.