Suppose I am walking along the street and meet a friend. I see him, speak to him, and pass on. But after I have passed on, I have ideas. I think of seeing my friend the day before. I think of what he said and of what he was doing, of what I said and of what I was doing. Perhaps for many minutes there come ideas from my past experience. These ideas were aroused by the perception of my friend. The perception was momentary, but it started a long train of memory ideas. I pass on down the street and go by a music store. Within the store, a victrola is playing _Jesus, Lover of My Soul_. The song starts another train of memory ideas. I think of the past, of my boyhood days and Sunday school, my early home and many scenes of my childhood. For several minutes I am so engrossed with the memory images that I scarcely notice anything along the street. Again, the momentary perception, this time of sounds, served to revive a great number of ideas, or memories, of the past. These illustrations are typical of our life. Every moment we have perceptions. These perceptions arouse ideas of our past life and experience. One of these ideas evokes another, and so an endless chain of images passes along. The older we become, the richer is our ideational life. While we are children, the perceptions constitute the larger part of our mental life, but as we become older, larger and larger becomes the part played by our memory images or ideas. A child is not content to sit down and reflect, giving himself up to the flow of ideas that come up from his past experience, but a mature person can spend hours in recalling past experience. This means that the older we grow, the more we live in the past, the less we are bound down by the present, and when we are old, instead of perceptions being the main part of mental life, they but give the initial push to our thoughts which go on in an endless chain as long as we live. =The Physiological Basis of Memory.= It will be remembered that the basis of perception is the agitation of the brain caused by the stimulation of a sense organ by an external thing or force. If there is no stimulation of a sense organ, there is no sensation, no perception. Now, just as the basis of sensation and perception is brain activity, so it is also the basis of ideas. In sensation, the brain activity is set up from without. In memory, when we have ideas, the brain activity is set up from within and is a fainter revival of the activity originally caused by the stimulation of the sense organ. Our ideas are just as truly conditioned or caused by brain activity as are our sensations. Memory presents many problems, and psychologists have been trying for many years to solve them. We shall now see what they have discovered and what is the practical significance of the facts.