Graham is a strong believer in UFOs, and he tends to assume that at any moment he is likely to spot a UFO in the sky. The other night he saw three streaks of light flash across the sky in a group. Graham saw the lights, and he thought they clearly looked exactly like 3 round UFOs. Graham presents his sighting as clear evidence for the existence of UFO's. Susan, who does not believe in UFOs, sees the same streaks of light and describes them as lights from airplanes. The difference between Graham's and Susan's perception of the strange lights in the sky illustrates the influence of: