The application of the methods described in Chapter 3 yields a quantity which is expressed in physical units and called the threshold. The concept of the threshold as an index of absolute and differential sensitivity has been extremely useful in the study of sensory systems. Through the use of this quantity, investigators have been able to discover the stimuli to which our sensory systems are the most and the least sensitive. But psychophysicists have not restricted their work to this descriptive level: they have also gone beyond this level to propose theories concerning the underlying mechanisms of sensory thresholds. Each theory was proposed to account for empirical data obtained in psychophysical experiments. The theorists hoped to describe the neurophysiological or psychological processes within the observer whieh may have determined the observer's behavior. The validity of each theory must be evaluated by determining the degree to which precise quantitative deductions from the theory are confirmed by experimental data.