The debate just described was a debate among philosophers, and it made few appeals to any sort of scientific evidence. Ultimately, though, the questions at stake could be understood as questions about whether our perceptions of the world reflect reality as it truly is, or instead reflect reality as it has been interpreted and categorized by us. These seem like questions that should be open to scientific scrutiny, and so it’s not too surprising that this dispute prodded investigators to explore in a more systematic way just how the senses function.
At the most basic level, this scrutiny must begin with the relationship between the physical inputs we receive—the stimuli—and the psychological experiences these stimuli give rise to. How closely do our experiences correspond to the inputs? Which inputs give rise to which experiences? The area of research that charts these relation-ships, linking psychological experiences to physical stimuli, is called psychophysics— an enterprise that asks questions like these: What will change in our perception of a sound as the frequency of the sound waves changes? What change in the physical attributes of light corresponds to the change from perceiving red to perceiving green? They might seem technical, but such questions are crucial if we are to understand the relationship between the objective, physically defined stimuli we encounter and the subjective, psychological world of our conscious experience. In other words, we’re try-ing to understand the relationship between the world as it actually is and the world as we perceive it to be.