Before conditioning, two distinctly unrelated stimuli are selected. Pavlov discovered that the meat powder was the stimulus that naturally produced/elicited or caused the salivation. Therefore, Pavlov referred to the meat powder as the Unconditioned Stimulus (UCS). When presented alone, the meat powder produced, elicited, and caused the Unconditioned Response (UCR). The natural salivation produced when the food powder was placed in the dog's mouth was the Unconditioned Response. The UCR is any response that is naturally caused and does not require learning.
During this phase, Pavlov was faced with demonstrating that the Conditioned Stimulus (the bell tone) did not initially elicit a Conditioned Response prior to learning or before being associated with the UCS. Therefore, Pavlov presented the Conditioned Stimulus (bell tone) alone and the dog did not salivate. At this point, the bell tone is actually a Neutral Stimulus (NS) which becomes the conditioned stimulus (CS).