Although the investigation of sensitivity by measuring absolute and difference thresholds provides valuable information about the senses, it does not in itself give a complete picture of a sensory system. If the input to a sensory system is the physical stimulus and the output is sensation, then all measurements in classical psychophysics were made on the input side of the system. Absolute and difference thresholds are not stated in sensation units but in units of stimulus energy at points where output can just be detected or where changes in outputs are just discriminable. An analogous situation is an engineer testing the sensitivity of a photo cell or sound level meter where certain changes in the output of the device are measured as a function of input changes. The absolute sensitivity of the device might be measured by determining the smallest amount of energy that will yield a meter reading. Differential sensitivity could be measured by determining the smallest changes in energy required to change the meter reading. Such measurements on the electronic device are useful, but obviously incomplete until the output of the device has also been measured and related to the input. If the device were a photo cell, we would complete the set of measurements by ·determining the output in voltage or current as a function of energy input.