The _association of ideas_, upon which is based the _unity_ of the continuous life of the individual, with the pervading sense of personal identity, has been aptly called the '_cohesion of the moral world_.' It is not less powerful, less irresistible, than that of the physical world. The association of ideas is a constituent and necessary phase of the _unity_ of our mental and moral being, the indispensable condition of all development, whether of mind or soul. Without the power of association, the intellect would strive in vain to construct consecutive trains of thought; it would indeed be condemned to eternal infancy, because, as it ascertained new relations, those already acquired would escape, and a labor constantly renewed would be requisite to regain them. Without association of ideas, no voluntary virtue would be possible; and at the end of long years of effort and self-restraint, we would have gained no additional control over the course of our impetuous passions. The fact that much of the difference in intellectual capacity so strongly characterizing different individuals arises from their various powers over the flow and logical association of ideas, has scarcely elicited the attention it so well deserves. It is of immense importance in the history of mental development. If an individual connects his ideas with difficulty, or can continue to chain them in rational sequence only with the most laborious efforts, he will have either a dull and heavy, or flighty and illogical, mind. If another has great trouble in modifying or arranging the association of ideas which arise spontaneously in the soul, he will suffer himself to be ruled by them, in place of exercising rational domination over them; he will pursue every chimera; he will trust every impulse; he will but dream, even when he tries to think; and will be of a weak and fickle, but obstinate and self-opinionated, intellect. His whole exhaustive logic will consist in clothing in exact and reiterated assertions the heterogeneous order in which ideas are arbitrarily, accidentally, and spontaneously associated in his own imagination. Another will associate his ideas in logical sequence, yet with startling rapidity; in a manner and through subtle relations quite unknown to common men, incapable of such vivid, rational, and consequential combinations; and will, in consequence, be a man of clear and vivid intellect.