With the intellect, however, the case is quite different. The intellect is neither debilitated nor stupefied by the discovery of truths that are exceptionally profound, or unusually abstruse, or strikingly evident; nor is it temporarily incapacitated thereby from understanding simpler, easier, or less evident truths. On the contrary, the more comprehensive, the more penetrating, the more perspicuous, the more sublime our intellectual vision is, so much the more is our intellect invigorated and enthused in its pursuit of truth, and its knowledge of the highest truths renders it not less, but more, apt for the understanding of simple and ordinary truths. Obviously, then, the intellect is not bound to a corruptible organ like the senses, but has for its subject a spiritual principle that is intrinsically independent of the organism. In opposition to this contention, it may be urged that a prolonged exercise of intellectual activity results in the condition commonly known as brain-fag. But this fatigue of the brain is not, as a matter of fact, the _direct_ effect of intellectual activity; rather it is the direct effect of the activity of the imagination, and only _indirectly_ the effect of intellectual thought. The intellect, as we have seen, requires a constant flow of associated and aptly coördinated imagery as the substrate of its contemplation. Now, the imagination, which supplies this imagery, is a cerebral sense, whose activity is directly proportionate to, and commensurate with, the metabolic processes at work in the cortical cells. Its exercise is directly dependent upon the energy released by the decomposition of the cerebral substance. Prolonged activity of the imagination, therefore, involves the destruction of a considerable amount of the cortical substance, and results in temporary incapacitation or paralysis of the imagination, which must then be compensated by a process of repair in the cortical neurons, before the imagination can resume its normal mode of functioning. Brain-fag, then, is due to the activity of the imagination rather than that of the intellect. That such is the case appears from the fact that after the initial exertion, which results from the imagination being forced to assemble an appropriate and systematized display of illustrative imagery as subject-matter for the contemplation of the intellect, the latter is henceforth enabled to proceed with ease along the path of a given science, its further progress being smooth and unhampered. Once the preliminary work imposed upon the imagination is finished, the sense of effort ceases and intellectual investigation and study may subsequently reach the highest degrees of concentration and intensity, without involving corresponding degrees of fatigue or depression on the part of the cerebral imagination, just as, conversely speaking, the activity of the cerebral imagination may reach degrees of intensity extreme enough to induce brain-fag in psychic operations