A mind is a system of ideas, each with the excitement it arouses, and with tendencies impulsive and inhibitive, which mutually check or reinforce one another.
 William James, The Varieties of Religious Experience (1902). copy citation

Context

“And our explanations then get so vague and general that one realizes all the more the intense individuality of the whole phenomenon. In the end we fall back on the hackneyed symbolism of a mechanical equilibrium. A mind is a system of ideas, each with the excitement it arouses, and with tendencies impulsive and inhibitive, which mutually check or reinforce one another. The collection of ideas alters by subtraction or by addition in the course of experience, and the tendencies alter as the organism gets more aged. A mental system may be undermined or weakened by this interstitial alteration just as a building is, and yet for a time keep upright by dead habit.” source