92. How is attention assisted by special muscular activity? 93. What causes the illusion that attention is a voluntary activity of the mind upon its contents? 94. What practical problems are connected with the law of the duration of attention? § 9. MEMORY While attention means limitation, memory means expansion. From the enormous number of impressions calling simultaneously for response, the mind selects a small group of those related to its present needs. But the mind may go beyond the limits of that which is presented and respond to impressions of a former time. We then speak of memory. When I hear the first verse of a poem which I have previously heard or read more than once, I continue to hear, in imagination, the following verses although the reader has stopped. When I see a black cloud drawing over the sky and the trees bowing under the pressure of the wind, I know that a thunderstorm is approaching. When I smell carbolic acid or iodoform, I look for a person wearing a bandage. In every case the mind tends toward expansion beyond the limits of the data presented at the moment. The mind thus restores the connections in which the accidentally isolated object of present interest has been experienced with other objects in the past. We refer to this ability of expansion by the term _memory_, to the actual process of expansion by _reproduction_ or _association_. The immense importance of memory for life is easily understood. Nature repeats itself--not without some variations of the accompanying phenomena; but no group of phenomena, aside from such variations, fails to recur at frequent intervals. In reproducing what previously existed under similar conditions, our mind possesses, as a rule, a real knowledge of what now exists but happens to remain hidden, and of what is about to occur. Thus our mind adapts itself to those parts of the world which are for spatial or temporal reasons beyond the reach of our sense organs. A special case of reproduction deserves to be mentioned because of its frequency of application. Two things may possess one common part while completely differing in other parts: for example, two words that rhyme, or a photograph and an oil portrait, or either of these and the face of the original. Let us call the parts of one thing _abcd_, those of another _cdef_. It easily happens that by mediation of the common parts, _cd_, the train of thought is carried from _ab_ to _ef_. Thus we may say that our train of thought is determined, not only by simultaneity of previous experience, which is often quite fortuitous, but also by similarity, by essential connection, by relationship.