What is always reckoned the mystery by pre-eminence is the union of BODY and MIND. How, then, should we treat this Mystery according to the spirit of modern thought, according to the modern laws of explanation? The course is to _conceive_ the elements according to the only possible plan, our own sensibility or consciousness; which gives us matter as one class of facts--extension, inertness, weight, and so on; and mind as another class of facts--pleasures, pains, volitions, ideas. The difference between these two is total, diametrical, complete; there is really nothing common to the experience of pleasure and the experience of a tree; difference has here reached its _acme_; agreement is eliminated; there is no higher genus to include these two in one; as the ultimate, the highest elements of knowledge, they admit of 110 fusion, no resolution, no unity. Our utmost flight of generality leaves us in possession of a double, a _couple_ of absolutely heterogeneous elements. Matter cannot be resolved into mind; mind cannot be resolved into matter; each has its own definition; each negatives the other. This being the fact, we accept it, and acquiesce. There is surely nothing to be dissatisfied with, to complain of, in the circumstance that the elements of our experience are, in the last resort, two, and not one. If we had been provided with fifty ultimate experiences, none of them having a single property in common with any other; and if we had only our present limited intellects, we might be entitled to complain of the world's mysteriousness in the one proper acceptation of mystery--namely, as overpowering our means of comprehension, as loading us with unassimilable facts. As it is, matter, in its commoner aspects and properties, is perfectly intelligible; in the great number and variety of its endowments or properties, it is revealed to us slowly and with much difficulty, and these subtle properties--the deep affinities and molecular arrangements--- are the mysteries rightly so called. Mind in itself is also intelligible; a pleasure is as intelligible as would be any transmutation of it into the inscrutable essence that people often desiderate. It is one of the facts of our sensibility, and has a great many facts of its own kindred, which makes it all the more intelligible. The varieties of pleasure, pain, and emotion are very numerous; and to know, remember, and classify them, is a work of labour, a _legitimate_ mystery. The subtle links of thought are also very various, although probably all reducible to a small number; and the ascertaining and following out of these has been a work of labour and time; they have, therefore, been mysterious; mystery and intellectual toil being the real correlatives. The _complications_ of matter and the _complications_ of mind are genuine mysteries; the reducing or simplifying of these complications, by the exertions of thinking men, is the way, and the only way out of the darkness into light.