Now if you ask of psychology just _how_ the excitement shifts in a man’s mental system, and _why_ aims that were peripheral become at a certain moment central, psychology has to reply that although she can give a general description of what happens, she is unable in a given case to account accurately for all the single forces at work. Neither an outside observer nor the Subject who undergoes the process can explain fully how particular experiences are able to change one’s centre of energy so decisively, or why they so often have to bide their hour to do so. We have a thought, or we perform an act, repeatedly, but on a certain day the real meaning of the thought peals through us for the first time, or the act has suddenly turned into a moral impossibility. All we know is that there are dead feelings, dead ideas, and cold beliefs, and there are hot and live ones; and when one grows hot and alive within us, everything has to re‐ crystallize about it. We may say that the heat and liveliness mean only the “motor efficacy,” long deferred but now operative, of the idea; but such talk itself is only circumlocution, for whence the sudden motor efficacy? 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. But a new perception, a sudden emotional shock, or an occasion which lays bare the organic alteration, will make the whole fabric fall together; and then the centre of gravity sinks into an attitude more stable, for the new ideas that reach the centre in the rearrangement seem now to be locked there, and the new structure remains permanent.