The phenomena of the Mind include multitudes of facts, of an extraordinary degree of complexity. By observing them one at a time with sufficient care, it is possible in the mental, as it is in the material world, to obtain empirical generalizations of limited compass, but of great value for practice. When, however, we find it possible to connect many of these detached generalizations together, by discovering the more general laws of which they are cases, and to the operation of which in some particular sets of {viii} circumstances they are due, we gain not only a scientific, but a practical advantage; for we then first learn how far we can rely on the more limited generalizations; within what conditions their truth is confined; by what changes of circumstances they would be defeated or modified. Not only is the order in which the more complex mental phenomena follow or accompany one another, reducible, by an analysis similar in kind to the Newtonian, to a comparatively small number of laws of succession among simpler facts, connected as cause and effect; but the phenomena themselves can mostly be shewn, by an analysis resembling those of chemistry, to be made up of simpler phenomena. "In the mind of man," says Dr. Thomas Brown, in one of his Introductory Lectures, "all is in a state of constant and ever-varying complexity, and a single sentiment may be the slow result of innumerable feelings. There is not a single pleasure, or pain, or thought, or emotion, that may not, by the influence of that associating principle which is afterwards to come under our consideration, be so connected with other pleasures, or pains, or thoughts, or emotions, as to form with them, for ever after, an union the most intimate. The complex, or seemingly complex, phenomena of thought, which result from the constant operation of this principle of the mind, it is the labour of the intellectual inquirer to analyse, as {ix} it is the labour of the chemist to reduce the compound bodies on which he operates, however close and intimate their combination may be, to their constituent elements.... From the very instant of its first existence, the mind is constantly exhibiting phenomena more and more complex: sensations, thoughts, emotions, all mingling together, and almost every feeling modifying, in some greater or less degree, the feelings that succeed it; and as, in chemistry, it often happens that the qualities of the separate ingredients of a compound body are not recognizable by us in the apparently different qualities of the compound itself,--so in this spontaneous chemistry of the mind, the compound sentiment that results from the association of former feelings has, in many cases, on first consideration, so little resemblance to these constituents of it, as formerly existing in their elementary state, that it requires the most attentive reflection to separate, and evolve distinctly to others, the assemblages which even a few years may have produced.