We have already trenched upon this factor of inductive method, the one that is the most important of all wherever it is feasible. Theoretically, one sample case _of the right kind_ will be as good a basis for an inference as a thousand cases; but cases of the "right kind" rarely turn up spontaneously. We have to search for them, and we may have to _make_ them. If we take cases just as we find them--whether one case or many cases--they contain much that is irrelevant to the problem in hand, while much that is relevant is obscure, hidden. The object of experimentation is the _construction, by regular steps taken on the basis of a plan thought out in advance, of a typical, crucial case_, a case formed with express reference to throwing light on the difficulty in question. All inductive methods rest (as already stated, p. 85) upon regulation of the conditions of observation and memory; experiment is simply the most adequate regulation possible of these conditions. We try to make the observation such that every factor entering into it, together with the mode and the amount of its operation, may be open to recognition. Such making of observations constitutes experiment. [Sidenote: Three advantages of experiment] Such observations have many and obvious advantages over observations--no matter how extensive--with respect to which we simply wait for an event to happen or an object to present itself. Experiment overcomes the defects due to (_a_) the _rarity_, (_b_) the _subtlety_ and minuteness (or the violence), and (_c_) the rigid _fixity_ of facts as we ordinarily experience them. The following quotations from Jevons's _Elementary Lessons in Logic_ bring out all these points: (_i_) "We might have to wait years or centuries to meet accidentally with facts which we can readily produce at any moment in a laboratory; and it is probable that most of the chemical substances now known, and many excessively useful products would never have been discovered at all by waiting till nature presented them spontaneously to our observation." This quotation refers to the infrequency or rarity of certain facts of nature, even very important ones. The passage then goes on to speak of the minuteness of many phenomena which makes them escape ordinary experience: (_ii_) "Electricity doubtless operates in every particle of matter, perhaps at every moment of time; and even the ancients could not but notice its action in the loadstone, in lightning, in the Aurora Borealis, or in a piece of rubbed amber. But in lightning electricity was too intense and dangerous; in the other cases it was too feeble to be properly understood. The science of electricity and magnetism could only advance by getting regular supplies of electricity from the common electric machine or the galvanic battery and by making powerful electromagnets. Most, if not all, the effects which electricity produces must go on in nature, but altogether too obscurely for observation."