He obtained the same null result with dogs and, even when he showed the animals the methods of opening a box by placing their paws on the appropriate levers and so on, he found no improvement. He fell back on a much simpler trial and error explanation of learning. Occasionally, quite by chance, an animal performs an action which frees it from the box. When the animal finds itself in the same position again it is more likely to perform the same action again. The reward of being freed from the box somehow strengthens an association between a stimulus, being in a certain position in the box, and an appropriate action.
Reward acts to strengthen stimulus-response associations.
The animal learns to solve the puzzle-box not by reflecting on possible actions and really puzzling its way out of it but by a quite mechanical development of actions originally made by chance. By 1910 Thorndike had formalised this notion into a 'law' of psychology - the law of effect. The greater the satisfaction or discomfort, the greater the strengthening or weakening of the bond . Thorndike maintained that, in combination with the law of exercise, the notion that associations are strengthen by use and weakened with disuse, and the concept of instinct, the law of effect could explain all of human behavior in terms of the development of myriads of stimulus-response associations. It is worth briefly comparing trial and error learning with classical conditioning .