Associative learning had been discussed for centuries, it remained, for Ivan Pavlov, to capture the phenomenon of classical conditioning. Pavlov repeatedly presented a neutral stimulus, just before an unconditioned stimulus, triggered an unconditioned response. After several repetitions, the stimulus alone began triggering a conditioned response. Further experiments on acquisition revealed that classical conditioning was usually greatest when the CS was presented just before the UCS, thus preparing the organism for what was comming.