Who is Ivan Pavlov?

He formulated what we call today classical conditioning. I don't think of it as a theory but a fact because it can be demonstrated experimentally. It was just called conditioning until B.F. Skinner came up with another kind of conditioning called operant conditioning. Conditioning in this sense is learning.

Pavlov showed that if you ring a bell every time a dog is fed, the dog will learn to associate the bell with eating an will salivate when the bell was rung even without the presence of food.