cessful pursuit of knowledge in any one of them requires some acquaintance with the others.
2. Methods.—The ultimate basis of all our knowledge of Nature is experience,—experience resulting from the action of bodies on our senses, and the consequent affections of our minds.
When a natural phenomenon arrests our attention, we call the result an observation. Simple observations of natural phenomena only in rare instances can lead to such complete knowledge as will suffice for a full understanding of them. An observation is the more complete, the more fully we apprehend the attending circumstances. We are generally not certain that all the circumstances which we note are conditions on which the phenomenon, in a given case, depends. In such cases we modify or suppress one of the circumstances, and observe the effect on the phenomenon. If we find a corresponding modification or failure with respect to the phenomenon, we conclude that the circumstance, so modified, is a condition. We may proceed in the same way with each of the remaining circumstances, leaving all unchanged except the single one purposely modified at each trial, and always observing the effect of the modification. We thus determine the conditions on which the phenomenon depends. In other words, we bring experiment to our aid in distinguishing between the real conditions on which a phenomenon depends, and the merely accidental circumstances which may attend it.
But this is not the only use of experiment. By its aid we may frequently modify some of the conditions, known to be conditions, in such ways that the phenomenon is not arrested, but so altered in the rate with which its details pass before us that they may be easily observed. Experiment also often leads to new phenomena, and to a knowledge of activities before unobserved. Indeed, by far the greater part of our knowledge of natural phenomena has been acquired by means of experiment. To be of value, experiments must be conducted with system, and so as to trace out the whole course of the phenomenon.
Having acquired our facts by observation and experiment we
