Extrasensory perception: set of phenomena called psi , divided into esp and psychokinesis. Psychokinesis is the ability to make things happen in the environment. (p. 232-) Its sort of like a toothache in my stomach . We are woefully poor at describing our sensations. The sub science of psychophysics is what we use to deal with these problems. (p. Psychophysics: the sensory consequences of controlled physical stimulation. We hope to be able to compare and make statements about humans senses. Absolute threshold: stimulus intensity detectable on 50% of its presentations. Not a constant value but changes from person to person and situation to situation. Difference threshold: what is the smallest difference between stimuli that we can detect. Weber"s law (1843): [delta]i/i = k (weber"s constant) for every stimulus, there is some constant percentage of stimulus intensity that must be added to or subtracted from that stimulus for a difference to be detected.