The following Treatise is an expansion of the Outline of Psychology contained in the Fourth and Fifth Editions of my “Principles of Human Physiology” (1852 and 1855), but omitted from the later editions of that work, to make room for new matter more strictly Physiological. The appreciation of that Outline expressed at the time by several friends to whose opinions I attached great value, made me contemplate the separate reproduction of it, at some future date, in an enlarged form: but the fulfilment of that intention has been delayed, in the first instance, by the pressure of Official duties; and, since this has been lightened, by the diversion of all the time and thought I could spare into an entirely different line of Scientific investigation. That investigation, however, having been taken in hand by Her Majesty's Government for systematic prosecution by the “Challenger” Expedition, I found myself free to entertain a proposal made to me by the projectors of the “International Scientific Series,” to republish my Outline in an enlarged form, as one of their Popular Treatises.

Not having seen reason to make any important change in my own Psychological views since I first put them forward, but, on the contrary, having found them confirmed and extended by the experience and reflection of twenty years, I set myself to revise my former exposition of them, with the idea of simply introducing such illustrations as might lead to the more ready apprehension of the principles I aimed to enforce, and of filling-up such deficiencies as it might seem most desirable to supply.