One thing about the information that comes in through the distance sense organs is that much of it does not require an immediate activity. For example, a hungry fox, seeing a fowl roosting in an accessible place, might pounce upon it at once, but if he happened at the moment to be either fully satisfied or to be carrying a fowl which he had captured a moment ago, there would be no occasion for him to seize this one; rather would it be desirable for him to set it aside in his mind to be captured in the future. In the case used in the illustration the nervous action goes on in the beginning just as we have been picturing it; that is, the sense organ is aroused and this in turn starts a disturbance over the nerve path; when it comes to the brain, however, it does not go on from there to cause immediate muscular activity, but instead is held in the brain and can cause activity at some future time, as for example if the fox, after taking the fowl which he is carrying home to his young, comes back in search of the one that he saw roosting. We are perfectly familiar with this ability to stop nervous disturbances; we know it under the name of _memory_, although we may not have thought of memory in exactly this sense. If we consider what memory really is, we shall see that it is just what we have been describing, namely the stopping of nervous disturbances that come in from the sense organs and holding them, so that they may in turn set up at some future time a nervous disturbance that shall produce activity. If we try to picture what actually goes on in the nerve cells where this memory is located, we can say only that when the nervous disturbance strikes upon these cells, it does not pass on through at once leaving them very much as they were before, but stops in them and brings about in them some kind of permanent change. An interesting thing about memory is its persistence. We may act upon memories that we have held for years. More than one person moving away from a particular town early in life, and going back to it in later years has remembered things he saw in childhood well enough to find his way to them again. Since memory is registered in the brain cells as a permanent or fairly permanent change, it can become the source of nervous disturbance over and over again. This in fact is one of its greatest advantages, because when we have once learned a thing, we can make use of it a great many times, and do not have to have the sense organs freshly aroused every time it is desirable to use this particular bit of information.