In contrast with the empirical method stands the scientific. Scientific method replaces the repeated conjunction or coincidence of separate facts by discovery of a single comprehensive fact, effecting this replacement by _breaking up the coarse or gross facts of observation into a number of minuter processes not directly accessible to perception_. [Sidenote: Illustration from _suction_ of empirical method,] If a layman were asked why water rises from the cistern when an ordinary pump is worked, he would doubtless answer, "By suction." Suction is regarded as a force like heat or pressure. If such a person is confronted by the fact that water rises with a suction pump only about thirty-three feet, he easily disposes of the difficulty on the ground that all forces vary in their intensities and finally reach a limit at which they cease to operate. The variation with elevation above the sea level of the height to which water can be pumped is either unnoticed, or, if noted, is dismissed as one of the curious anomalies in which nature abounds. [Sidenote: of scientific method] [Sidenote: Relies on differences,] Now the scientist advances by assuming that what seems to observation to be a single total fact is in truth complex. He attempts, therefore, to break up the single fact of water-rising-in-the-pipe into a number of lesser facts. His method of proceeding is by _varying conditions one by one_ so far as possible, and noting just what happens when a given condition is eliminated. There are two methods for varying conditions.[24] The first is an extension of the empirical method of observation. It consists in comparing very carefully the results of a great number of observations which have occurred under accidentally _different_ conditions. The difference in the rise of the water at different heights above the sea level, and its total cessation when the distance to be lifted is, even at sea level, more than thirty-three feet, are emphasized, instead of being slurred over. The purpose is to find out what _special conditions_ are present when the effect occurs and absent when it fails to occur. These special conditions are then substituted for the gross fact, or regarded as its principle--the key to understanding it. [24] The next two paragraphs repeat, for purposes of the present discussion, what we have already noted in a different context. See p. 88 and p. 99. [Sidenote: and creates differences]