The beginnings of behaviourism overlap in terms of time with psychodynamics, as both started and grew during the first two decades of the 20th century.However, they are complete opposites in content; while the psychodynamic psychologists considered the unconscious mind to be the irrational driving force behind almost all behaviour, behaviourists ignore it completely.
The main principle behind Behaviourism is that every behaviour we do is learnt from our environments, as a stimulus in the environment causes a response from us. These S-R bonds rely on the concept of “conditioning”- the association between two stimuli in classical conditioning, and the association between a stimulus and a response in operant conditioning. However, the actual internal processes that link them is ignored, and seen as the mysterious and un-knowable “black box” connecting stimuli and responses.
The first psychologist to explain the concept of conditioning was Thorndike, who began his career studying the intelligence of cats by making them escape boxes by solving puzzles or pulling levers.
By watching how they responded to new boxes by attempting the same skills they had learnt in previous boxes, and their reactions to success or failure, he created the law of effect. This states; “responses that produce a satisfying effect in a particular situation become more likely to occur again in that situation, and responses that produce a discomforting effect become less likely to occur again in that situation.” in other words, if you succeed at something, you’re a lot more likely to do it again than if you failed. This was then later formed into Operant Conditioning by B.F Skinner.
After Thorndike had studied animals, he put the knowledge about learning he had gained into developing strategies for human learning, therefore becoming one of the first educational psychologists. (and technically one of the first comparative psychologists as well). His theory of learning was still based on associations, and also on the principle that the ability to lean did not decrease with age; he believed there was no decline in learning ability until age 35, only in learning speed.
His future works were very diverse including developing the Army entrance tests, writing a book on the psychology of maths, and helping develop Interlingua, one of the first Constructed languages. So he was, like all of the early psychologists seem to be, a bit of a polymath.