"Intuitive." We use the word a lot when talking about interactive systems, but it misleads us.

When people say they want a system to be "intuitive," they typically think they mean that users should immediately understand how a system works when they encounter it. But you cannot really do that with many systems … not even with most systems people talk about when you ask them for an example of something "intuitive."

Consider the mouse-and-cursor. Most of us have forgotten the first time we encountered it, and thus forgotten how unintuitive we found it the first time we used it. A little box on a string with a button or three on top? If you have just arrived from the 23rd century, you might pick it up and try talking to it. But with ten seconds of demonstration you understand it completely and have some sophisticated applications of it immediately available to you, and even if you didn't see a mouse again for the next ten years you would still remember how it worked.

There you have what people really mean by "intuitive:" easy to explain, powerful in its implications, impossible to forget. You get that through systems that possess a clear, coherent internal logic that feels natural and obvious. Of course, it can take hard work to figure out those "natural and obvious" behaviors; we interaction designers call that work "interaction design."

From time to time the talk at Cooper turns to the deeper philosophical waters of our work. Some years ago, we discussed the ultimate objectives of our work, and the expressions "intuitive" and "easy-to-use" started coming up, but didn't quite hit the mark. Some systems have an inherent difficulty; having worked on tools for doctors and NMR spectroscopers and rocket scientists (yeah, really) we have seen how some systems can never really become "easy-to-use."

Now in a lot of cases you can improve an interactive system by simplifying it, stripping things out, and thus making it easier to use and understand. Most interactive products and services out there could benefit from this treatment, which can paradoxically result in the simplified version delivering more functionality to users because they understand it better and take better advantage of it.

This has its limits. You can't make that rocket scientist's satellite orbit plotting system "easy." But you can make it "intuitive." With that comes a surprise benefit.

Remember when a lot of folks started taking to the Palm Pilot? It seemed like everywhere you went, people were tinkering with their Palms all of the time, obnoxiously wanting to share with you, "look at how you can do this!" Why?

Because the Palm has "intuitive" interaction design, which makes using it pleasurable.

I believe that this hits us at a deep, animal level. Just as we get pleasure from the form and tactility of good industrial design, we get pleasure from good interaction design, both as we learn it and as we work with it. Learning things that make sense, working with tools that work right; these things make us East African Plains Apes happy right down to our DNA. So instead of saying "intuitive" or "easy-to-use," at Cooper we often talk about designing interactive products that deliver power and pleasure to the people who use them.

With so many folks talking about the iPhone right now, it surprises me how few people remark on the effect of its lovely interaction design. Notice the difference between the advertisements for the iPhone and every other phone on the market. Other phones show people talking on the phone, snapping pictures of each other, dancing around, getting into kung fu battles with phones as weapons, everything except what the iPhone ads show you: how you use the phone. Think about that. They sell the iPhone by showing you thirty seconds of someone using it, assuming that this will make people want it … which it does, because people recognize that using a product with good interaction design feels good.

Part of that lies in the harmonious way that the iPhone transitions between its functions, the thing that folks often call "navigation," because finding your way between functions in a system with bad interaction design can feel like navigating your way through a maze or a trackless sea. Another aspect that I find particularly interesting lies in the iPhone's gestural interface.

A few key functions rely not just on touching the phone, but dragging your fingers over it: flipping through pictures and song lists, resizing pictures, and the cunning mechanism for unlocking the phone. This really struck me because for the past few years, instead of a mouse I have been using an exotic trackpad made by a now-defunct company called FingerWorks.

My trackpad looks a bit like an ordinary mouse pad, and like the iPhone's photo-resizing function it can recognize when I touch it with more than one finger, so it does more than just move my cursor. By pinching my fingers in a certain way, I can cut something to the clipboard; if I reverse the gesture, I can paste it back in somewhere else. A dozen other gestures enable me to do things like open new documents, close windows, scroll, resize things, and jump to my home page in my web browser. Colleagues say that sometimes when they see me work it looks like I have cast some magic spell: I wave my hand and things happen on the screen.

When I use an ordinary computer I miss my trackpad, and not only because it provides me with a little convenience. It also feels good.

I believe that part of why I enjoy it so much comes from something we don't get to do much when working with computers and other interactive tools: I do the gestures from muscle memory, rather than cognitive memory, just like I do with my typing on my computer keyboard. Most of the time tools that run on software tax our cognitive capacity but leave the intelligence that lives in our bodies relatively untapped, which makes us East African Plains Apes a little uncomfortable; using those gestures makes me a happier animal.

I think many of us know on some level that we suffer this imbalance, which explains why we ooh and aah over things like tablet computers, Jeff Han's big touchscreen, Microsoft's Surface table, and the computer from Minority Report. Even if these examples seem unrealistic in some ways—I have a hard time imagining what anyone would actually use the fantastically expensive MS Surface for, and I assume that Tom Cruise's character in Minority Report has such broad shoulders because the poor guy has to wave his arms around all day just to use his computer—they appeal to us because we yearn for the pleasures of using the intelligence of our bodies together with the intelligence that lies between our ears.

I wondered for a long time whether I was right to suspect that using gestural input like on my trackpad really would appeal to most people, or whether it just reflected an eccentricity of mine. The iPhone seems to have demonstrated that gestures really do have a broad appeal. I hope that companies other than Apple learn the lesson, if only because I want to use—and get to design—the products that will include them.