Our most basic insight about UI design is the Rule of Bliss: ignorance is a luxury because human beings are happiest when they can handle routine tasks without having to use logic or allocate any of their relatively limited capacity for handling novelty. The kind of cognitive consciousness that we summon to deal with novelty and branching decisions is, as Jef Raskin reminds us, normally only sustainable for seconds at a time; unconscious cognition and adaptive habit are the more normal state.

The next simplest insight about how humans interact with UIs may be the most important, which we'll call Raskin's First Law because Raskin (op. cit.) has discussed its consequences in more depth than anyone else:

Raskin's First Law: Most human beings can only concentrate on one thing at a time.

The pattern of human competence suggests that our ancestral environment shaped us for an regime of sporadic but intense challenges, one in which nothing much happens most of the time but we have to be at maximum during the few minutes per average day when we are either predator or prey. Under stress, our attention becomes more single-focused. But evolution has not made us any more capable of sustained attention than we have to be to cope with transient crises, because excess capacity is inefficient.

Despite scattered results suggesting (for example) that under some circumstances humans can have two simultaneous loci of spatial attention, the actual measured task performance of human beings on computers suggests that Raskin is correct when he maintains that in the GUI context there is one single locus of attention, and that managing that locus effectively without jerking it around is a problem right at the center of GUI design.

Natural settings like our environment of ancestral adaptation also differ from technological civilization in that they may require an organism to react quickly, or to cope with novelties outside its instinctive responses, but they seldom do both at once. Thus a related and equally important result:

Raskin's Second Law: Humans take up to ten seconds to prepare for tasks requiring conscious cognition, during which they are unaware of passing time.

This gap in subjective time has odd consequences. Raskin was able to make use of it in the design of the Canon Cat word processor by making a capture of the screen part of the standard disk save, then throwing that capture on the display at the begining of the document-load sequence, which (using some tricks) he held just below 7 seconds in duration. Most users perceived the reload as instantaneous, because it all happened during the 10-second subjective-time gap while they were mentally tooling up to work on their document.

The costs of focus-switching usually life more difficult for UI designers rather than easier. A major one is that users are prone to not notice interface friction caused by focus-switching, even when lost time is substantial. Macintosh UI guru Bruce Tognazzini has observed that users often think that a mousing through a GUI is quicker and more efficient than using keyboard shortcuts, even when actual timings show the reverse is true. The problem is that the multiple focus switches involved in finding and moving the mouse do not reach the user's awareness.