AKA "The Pavlonian Reinforcement". Possibly one of the most important psychological principles you can learn to overcome fear and negative mindsets for good and build CONFIDENCE. The neutral stimulus (NS) is what has no mental reaction, but paying attention to your environment. The unconditional stimulus (US) is what automatically causes your body to react to certain emotions (think anxiety, fear, laughter, happiness, nervousness, sad). The NS turns into the conditional stimulus (CS) when the NS collides with the US. The reaction produced from these 2 stimuli is what's called "The conditioned response".
Here is an example of classical conditioning at its finest...
Let's say you're at the mall or at the store (Neutral Stimulus). All of a sudden, you spot a chick you'd like approach, talk to and meet. But then, the sign of that chick gives you Approach Anxiety (Unconditional stimulus). Next, you get the guts to approach her, talk to her, and kiss her at the end, BUT she either rejects you or asks you if you're crazy or there's anything wrong with you (NS meets US, now NS turns to CS and it produces the conditioned response of a rapid heartbeat, trembling hands, and xerostomia which is cotton mouth). The more this process is continued which is habituation, the more you weaken the conditioned response to feel that icky sensation when you want to make your moves on a girl you like. TRUST ME... you'll feel A LOT more confident at the end!!