The mind, it is evident, may, like the body to which it is united, or the material objects which surround it, be considered simply as a substance possessing certain qualities, susceptible of various affections or modifications, which, existing successively as momentary states of the mind, constitute all the phenomena of thought and feeling. The general circumstances in which these changes of state succeed each other, or, in other words, the laws of their succession, may be pointed out, and the phenomena arranged in various classes, according as they may resemble each other, in the circumstances that precede or follow them, or in other circumstances of obvious analogy. There is, in short, a science that may be termed _mental physiology_, as there is another science relating to the structure and offices of our corporeal frame, to which the term _physiology_ is more commonly applied; and as, by observation and experiment, we endeavour to trace those series of changes which are constantly taking place in our material part, from the first moment of animation to the moment of death; so, by observation, and in some measure also by experiment, we endeavour to trace the series of changes that take place in the mind, fugitive as these successions are, and rendered doubly perplexing by the reciprocal combinations into which they flow. The innumerable changes, corporeal and mental, we reduce, by generalizing, to a few classes; and we speak, in reference to the mind, of its faculties or functions of perception, memory, reason, as we speak, in reference to the body, of its functions of respiration, circulation, nutrition. This mental physiology, in which the mind is considered simply as a substance endowed with certain susceptibilities, and variously affected or modified in consequence, will demand of course our first inquiry; and I trust that the intellectual analyses, into which we shall be led by it, will afford results that will repay the labour of persevering attention, which they may often require from you.