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and the message. Just as in doing an action we can distinguish between knowing what can be done and knowing how to do it, in reminding we must distinguish between knowing that something is to be remembered and remembering what it is. Most popular reminding devices provide only one of these two critical aspects. The famous “tie a string around your finger” reminder provides only the signal. It gives no hint of what is to be remembered. Writing a note to yourself provides only the message; it doesn’t remind you ever to look at it. (Tying a knot in your handkerchief—Carelman’s device in figure 3.2—provides neither signal nor message.) The ideal reminder has to have both components: the signal that something is to be remembered, the message of what it is.

3.2 Carelman’s Preknotted Handkerchief. What an aid to the forgetful—except that the act of tying the knot is probably just as useful a memory cue as the knot itself. (Jacques Carelman: “Preknotted Handkerchief” Copyright © 1969-76-80 Jacques Carelman and A. D. A. G. P. Paris. From Jacques Carelman, Catalog of Unfindable Objects. Balland, éditeur, Paris-France. Used by permission of the artist.)

The need for timely reminders has created loads of products that make it easier to put the knowledge in the world—alarm clocks, diaries, calendars. A variety of sophisticated watches and small, calculator-sized reminding devices are starting to appear. So far they are limited in power and difficult to use. But I believe there is a need for them. They just need more work, better technology, and better design.

Would you like a pocket-size device that reminded you of each appointment and daily event? I would. I am waiting for the day when portable computers become small enough that I can keep one with me at all times. I will definitely put all my reminding burdens upon it. It has to be small. It has to be convenient to use. And it has to be relatively powerful, at least by today’s standards. It has to have a full, standard typewriter keyboard and a reasonably large display. It needs good graphics, because that makes a tremendous difference in usability, and a lot of memory—a huge amount, actually. And it should be easy to hook up to the telephone; I need to connect it to my home and laboratory computers. Of course, it should be relatively inexpensive.

What I ask for is not unreasonable. The technology I need is available today. It’s just that the full package has never been put together, partly because the cost in today’s world would be prohibitive. But it will exist in imperfect form in five years, possibly in perfect form in ten.

NATURAL MAPPINGS

The arrangement of burners and controls on the kitchen stove provides a good example of the power of natural mappings to reduce the need for information in memory. Without a good mapping, the user cannot readily determine which burner goes with which control. Consider the standard stove with four burners, arranged in the traditional rectangle. If the four controls were truly arbitrary, as in figure 3.3, the user would have to learn each control separately: twenty-four possible arrangements. Why twenty-four? Start with the leftmost control: it could work any of the four burners. That leaves three possibilities for the next leftmost. So there are 12 (4 × 3) possible arrangements of the first two controls: four for the first, three for the second. The third control could work either of the two remaining burners, and then there is only one burner left for the last control. This makes twenty-four possible mappings between the controls and burners: 4 × 3 × 2 × I = 24. With the completely arbitrary arrangement, the stove is unworkable unless each control is fully labeled to indicate which burner it controls.

Most stoves have controls arranged in a line, even though the burners are arranged rectangularly. Controls are not mapped naturally to burners. As a result, you have to learn which control goes with which burner. Consider how the use of spatial analogies can relieve the memory burden. Start with a partial mapping that