The contents of this work are devoted almost exclusively to discussing the principles and the reasoning appropriate to the theoretical department of Natural Philosophy, and the mutual relation between this and the experimental department. The discoveries in recent times of new facts and physical laws by experimental means have been so remarkable and abundant, and have given rise to so much speculation, that there seemed to be reason to apprehend that the part of philosophy which is properly theoretical might be either set aside or wholly misunderstood. The purpose of this Essay is to endeavour to counteract this tendency.