But this distinction, in the first place, is not consistent with Timæus--wherein Plato assigns to each of his three human souls a separate and special region of the bodily organism, as its physical basis. Nor, in the second place, is it consistent with that larger range of observed facts which the farther development of physiology has brought to view. To Plato and Aristotle the nerves and the nervous system were wholly unknown: but it is now ascertained that the optic, auditory, and other nerves of sense, are only branches of a complicated system of sensory and motory nerves, attached to the brain and spinal cord as a centre: each nerve of sense having its own special mode of excitability or manifestation. Now the physical agency whereby sensation is carried on, is, not the organ of sense alone, but the cerebral centre acting along with that organ: whereas in the intellectual and memorial processes, the agency of the cerebral centre and other internal parts of the nervous system are sufficient, without any excitement beginning at the peripheral extremity of the special organ of sense, or even though that organ be disabled. We know the intelligent mind only in an embodied condition: that is, as working along with and through its own physical agency. When Plato, therefore, says that the mind thinks, computes, compares, &c., by itself--this is true only as signifying that it does so without the initiatory stimulus of a special organ of sense; not as signifying that it does so without the central nervous force or currents--an agency essential alike to thought, to sensation, to emotion, and to appetite. [Side-note: Plato's discussion of this question here exhibits a remarkable advance in analytical psychology. The mind rises from Sensation, first to Opinion, then to Cognition.]