The central insight of cognitive therapy as originally formulated over three decades ago is that thoughts mediate between stimuli, such as external events, and emotions. As in the figure below, a stimulus elicits a thought -- which might be an evaluative judgment of some kind -- which in turn gives rise to an emotion. In other words, it is not the stimulus itself, which somehow elicits an emotional response directly, but our evaluation of or thought about that stimulus.
Cognitive therapy aims to help the client to become aware of thought distortions, which are causing psychological distress, and of behavioural patterns, which are reinforcing it, and to correct them. The objective is not to correct every distortion in a client's entire outlook -- and after all, virtually everyone distorts reality in many ways -- just those that may be at the root of distress. The therapist will make every effort to understand experiences from the client's point of view, and the client and therapist will work collaboratively with an empirical spirit, like scientists, exploring the client's thoughts, assumptions and inferences. The therapist helps the client learn to test these by checking them against reality and against other assumptions