timelets

For example, consider Pavlov with his famous experiment: you show a dog a piece of meat while you ring a bell, and the dog salivates; after a week, you only ring the bell and the dog salivates because he has been conditioned, as one says, to take the independent stimulus for the meat. But some time later, a Polish experimental psychologist repeated the Pavlov experiments with a slight variation: he took the clapper off of the bell so it could not ring, but the assistant did not know that, so he stepped forward, took the bell ringing it - silence, and the dog salivated. One could say that the ringing of the bell was a stimulus for Pavlov, but not for the dog.

H.v.Foerester. Understanding Understanding, 2002. pp viii - ix.