The method of analysis by comparing cases is, however, badly handicapped; it can do nothing until it is presented with a certain number of diversified cases. And even when different cases are at hand, it will be questionable whether they vary in just these respects in which it is important that they should vary in order to throw light upon the question at issue. The method is passive and dependent upon external accidents. Hence the superiority of the active or experimental method. Even a small number of observations may suggest an explanation--a hypothesis or theory. Working upon this suggestion, the scientist may then _intentionally_ vary conditions and note what happens. If the empirical observations have suggested to him the possibility of a connection between air pressure on the water and the rising of the water in the tube where air pressure is absent, he deliberately empties the air out of the vessel in which the water is contained and notes that suction no longer works; or he intentionally increases atmospheric pressure on the water and notes the result. He institutes experiments to calculate the weight of air at the sea level and at various levels above, and compares the results of reasoning based upon the pressure of air of these various weights upon a certain volume of water with the results actually obtained by observation. _Observations formed by variation of conditions on the basis of some idea or theory constitute experiment._ Experiment is the chief resource in scientific reasoning because it facilitates the picking out of significant elements in a gross, vague whole. [Sidenote: Analysis and synthesis again]