Around the 1900's Edward Thorndike attempted to develop an objective experimental method for the mechanical problem solving ability of cats and dogs. Thorndike devised a number of wooden crates which required various combinations of latches, levers, strings and treadles to open them. A cat or dog would be put into one of these "puzzle-boxes" and sooner or later would manage to escape from it. Thorndike's initial aim was to show the anecdotal achievements of cats and dogs could be replicated in controlled, standardised circumstance. However, he soon realised he could now measure animal intelligence with this equipment. His method was to set an animal the same task repeatedly, each time measuring the time it took to solve it. He was particularly interested in discovering whether his animals could learn tasks through imitation or observation. He compared the learning curves of cats who had been given the opportunity of observing others escaping from a box with those who had never seen the box being solved and found no difference in their rate of learning. He obtained the same null result with dogs and even when he put the animal's paws on the appropriate lever and showed them how to open the box, there was no improvement. He fell back on a much simpler trial and error explanation of learning. He realised that when the animal performed the actions that opened the box they were more likely to perform the same action when put back in the box.
The behaviourist position that human behaviour could be explained completely in terms of reflexes, stimulus-response (SR) associations and the effect of reinforcers upon them entirely excluding "mental" terms like desires and goals was taken up by John Watson in his 1914 book "Behaviour: An Introduction to Comparative Psychology". Watson was also involved with the introduction of the laboratory rat to psychology. He was employed as caretaker to fund his studies and one of his duties was to look after the lab rats.