It may be important here to notice a common fault in the method frequently adopted by philosophers in their investigations in this department of mental science. In the most celebrated treatise that has ever appeared upon this subject, the writer does not recollect to have met with a single appeal to _consciousness_, the only adequate witness in the case. The whole treatise, almost, consists of a series of syllogisms, linked together with apparent perfectness, syllogisms pertaining to an abstract something called Will. Throughout the whole, the facts of consciousness are never appealed to. In fact, in instances not a few, among writers of the same school, the right to make such an appeal, on the ground of the total inadequacy of consciousness to give testimony in the case, has been formally denied. Would it be at all strange, if it should turn out that all the fundamental results of investigations conducted after such a method, should be wholly inapplicable to _the_ Will, the phenomena of which lie under the eye of consciousness, or to stand in plain contradiction to the phenomena thus affirmed? What, from the method adopted, we see is very likely to take place, we find, from experience, to be actually true of the treatise above referred to. This is noticed by the distinguished author of The Natural History of Enthusiasm, in an Essay introductory to Edwards on the Will. "Even the reader," he says, "who is scarcely at all familiar with abstruse science, will, if he follow our author attentively, be perpetually conscious of a vague dissatisfaction, or latent suspicion, that some fallacy has passed into the train of propositions, although the linking of syllogisms seems perfect. This suspicion will increase in strength as he proceeds, and will at length condense itself into the form of a protest against certain conclusions, notwithstanding their apparently necessary connection with the premises." What should we expect from a treatise on mental science, from which the affirmations of consciousness should be formally excluded, as grounds of any important conclusions? Just what we find to be true, in fact, of the above named treatise on the Will; to wit: all its fundamental conclusions positively contradicted by such affirmations. What if the decisions of our courts of justice were based upon data from which the testimony of all material witnesses has been formally excluded? Who would look to such decisions as the exponents of truth and justice? Yet all the elements in those decisions may be the necessary logical consequents of the data actually assumed. Such decisions may be all wrong, however, from the fact that the data which ought to be assumed in the case, were excluded. The same will, almost of necessity, be true of all treatises, in every department of mental science, which are not based upon the facts of consciousness. PROPER METHOD OF REASONING FROM REVELATION TO THE SYSTEM OF MENTAL PHILOSOPHY THEREIN PRE-SUPPOSED.