In considering the Phenomena of our Mind, as they exist when we are capable of making them subjects of reflection, I mentioned to you, in a former Lecture, that although we have to encounter many additional difficulties, in consequence of early associations, that modify forever after our original elementary feelings, with an influence that is inappreciable by us, because it is truly unperceived, there are yet some advantages, which though they do not fully compensate this evil, at least enable us to make some deduction from its amount. The benefit to which I allude, is found chiefly in the class of phenomena which we are now considering,--a class, indeed, which otherwise we should not have regarded as half so comprehensive as it truly is, since, but for our previous belief of the existence of a permanent and independent system of external things acquired from other sources, we should have classed by far the greater number of the feelings, which we now refer to _sense_, among those which arise spontaneously in the mind, without any cause external to the mind itself. Though the sensations, which arise from affections of the same organ--as those of warmth and extension for example, or at least the feeling of warmth and a tactual feeling, that is commonly supposed to involve extension, from affections of the same nerves of touch,--are not, in every case, more analogous to each other, than the sensations which arise from affections of different organs,--and though, if we were to consider the sensations alone, therefore, without reference to their organs, we might not form precisely the same classification as at present,--the division, according to the organs affected, in most cases corresponds, so exactly, with that which we should make, in considering the mere sensations as affections of the mind, and affords in itself a principle of classification, so obvious and definite, that we cannot hesitate, in preferring it to any other which we might attempt to form. In the arrangements of every science, it is of essential consequence, that the lines of difference, which distinguish one class from another, should be well marked; and this advantage is peculiarly important in the science of mind, the objects of which do not, as in the other great department of nature outlast inquiry, but are, in every case, so very shadowy and fugitive, as to flit from us, in the very glance, that endeavours to catch their almost imperceptible outline.