What is essential for conditioning to occur in classical conditioning?

In classical conditioning, the essential component for the conditioning to occur is the immediate presentation of the conditioned stimulus (CS) and the unconditioned stimulus (US) in sequence. The CS is a previously neutral stimulus that, when paired with the US (which naturally evokes a response), begins to elicit a similar response on its own. This pairing is typically most effective when the CS is presented shortly before or simultaneously with the US, allowing the subject to make an association between the two stimuli.

This immediate sequence is critical because it strengthens the connection between the CS and the US, leading to the conditioned response. If there is too much time between the presentation of the two stimuli, the association weakens, and conditioning is less likely to occur. The timing and order of the stimuli are therefore pivotal in establishing the conditioned response.