Association in psychology refers to a connection between conceptual entities or mental states that results from the similarity between those states or their proximity in space or time. The idea stems from Plato and Aristotle, especially with regard to the succession of memories, and it was carried on by philosophers such as John Locke, David Hume, David Hartley, James Mill, John Stuart Mill, and Alexander Bain. It finds its place in modern psychology in such areas as conditioning and in neural network models of memory.