Good user interface design is based on the realities of human psychology, which has evolved over eons. Jeff Johnson talks about the relationships between technology and human perception. In our daily use of computers we continually form goals, make plans to achieve them, and evaluate our progress, all in a fraction of a second. We consciously experience very little of this of decision-making in our daily interactions.

User interface design must take into account elements like color relationships and association with other sensory cues. Short-term and long-term memory affect design, in that recognition is easy but recall is hard. Seeing and choosing is easier than remembering and typing, so clicking icons and menus is more successful than typing commands. Johnson says software should not make the user deduce task completion. It is better to tell people what steps are involved because puzzling over steps in a process will tend to frustrate the user.

Jeff Johnson uses vivid examples to explain the perceptual constraints that good designers must keep in mind.