ABSTRACT
When an idea enters the mind it does not remain an independent unit for very long. It seems to be a fundamental characteristic of ideas that they form alliances among themselves, and these groups of ideas are technically known as complexes. A complex may be compared to the branching growth of a pond-weed; it has a central starting-point from which ramify threads that divide and subdivide, and branch in every direction, and connect it with other systems of ideas that have similar branching threads. Ideas tend to group themselves in complexes according to certain well-defined principles. All ideas connected with the same subject tend to become associated together. Ideas which enter the mind at the same time tend to become associated together. For instance, if have a nasty fall on a piece of banana skin while going to the pillar-box, when see bananas shall think of falls and pillar-boxes, and when see pillar-boxes, may think of bananas and falls.
