We must distinguish carefully the various forms of knowledge, and trust only the best. First, then, there is hearsay knowledge, by which, for example, I know the day of my birth. Second, vague experience, “empirical” knowledge in the derogatory sense, as when a physician knows a cure not by any scientific formulation of experimental tests, but by a “general impression” that it has “usually” worked. Third, immediate deduction, or knowledge reached by reasoning, as when I conclude to the immensity of the sun from seeing that in the case of other objects distance decreases the apparent size.
