Among Thorndike's most famous contributions were his research on cats escaping from puzzle boxes, and his formulation of the Law of Effect (among other Laws of learning, many now forgotten and some discredited).
The puzzle box experiments were motivated in part by Thorndike's dislike for statements that animals made use of extraordinary factulties such as insight in their problem solving: "In the first place, most of the books do not give us a psychology, but rather a eulogy of animals. They have all been about animal intelligence, never about animal stupidity." (Animal Intelligence, 1911). (This same opinion was applied to human beings by John B. Watson soon after.)
Thorndike meant to distinguish clearly whether or not cats escaping from puzzle boxes were using insight. Thorndike's instruments in answering this question were 'learning curves' revealed by plotting the time it took for an animal to escape the box each time it was in the box. He reasoned that if the animals were showing 'insight,' then their time to escape would suddenly drop to a negligible period, which would also be shown in the learning curve as an abrupt drop; while animals using a more ordinary method of trial and error would show gradual curves. His finding was that cats consistently showed gradual learning.
Thorndike interpreted the findings in terms of associations. He asserted that the connection between the box and the motions the cat used to escape was 'strengthened' by each escape. A similar, though radically reworked idea was taken up by B.F. Skinner in his formulation of "Operant Conditioning," and the associative analysis went on to figure largely in behavioral work through mid-century, now evident in some modern work in behavior as well as modern connectionism.