How do sensations or ideas become linked in the mind?

Aristotle identified the basic requirements for association more than 2000 years ago: contiguity, frequency and similarity. Pavlov showed how we can study and measure learning about associations that exist in the world. Thorndike showed how reward and punishment govern which associations we learn to make. Both Hull and Skinner built on the work of Thorndike, with Hull focusing on mathematical models to explain the favors that influence learning and Skinner expanding the experimental analyses of reward and punishment and applying his research to society. Today, most psychologists take for granted the idea that memory involves forming associations among ideas or sensations, although there are still many arguments about exactly how these associations are formed and how they are used