[35] The following is a legitimate question: To what kind of nervous processes is the sensation green to be mentally added. Such questions can be solved only by special inquiry, and not by a reference in a general way to motion and electric currents. How disadvantageous our remaining satisfied with such general conceptions is, can be seen from the fact that inquirers have been repeatedly on the brink of abandoning the _specific energies_, one of the greatest acquisitions we have made, simply because they were unable to discover any difference in the currents of different sensory nerves. I was impelled as early as 1863 in my lectures on psycho-physics to call attention to the fact that the _most diverse kinds_ of nervous processes can conceal themselves in a current. Current is an abstraction and places in relief but one feature of the process—the passage of energy though a transverse section. A current in diluted sulphuric acid is something entirely different from a current in copper. We must therefore also expect that a current in the acoustic nerve is something entirely different from a current in the optic nerve. [36] It is the transitoriness of sense-perceptions that so easily leads us to regard them as mere appearances as contrasted with permanent bodies. I have repeatedly pointed out that unconditioned permanent states do not exist in nature, that permanences of connection only exist. A body is for me the same complex of sight-and-touch-sensations every time that it is placed in the same circumstances of illumination, position in space, temperature, and so forth. The supposed constancy of the body is the constancy of the union of _A_, _B_, _C_ ... or the constancy of the _equation f_(_A_, _B_, _C_ ...) = 0. PROFESSOR CLIFFORD ON THE SOUL IN NATURE. No one can read Clifford’s Lectures and Essays, without feeling that, if their author is less known and valued as an original thinker than as a master of mathematical analysis, it is only because having turned the force of his genius onto mathematics first he had time to complete some work in that direction, whereas his premature death in 1879 only allowed him to give us an earnest of the philosophical work which he had it in him to perform.