Aza Raskin artfully unpacks some assumptions regarding the concepts of home and back for a mobile device.
If you sit and watch people use an iPhone there’s a mistake made often and reliably: They hit the home button when they mean to just go back to the app’s main screen. Going home has heavy consequences—to recover you’ve got to find that app again, sit through its splash screen, and fiddle the app to where it was before. The home button is the grunt-and-touch control of physical affordances. While iconically simple, the one bit of information it lets you indicate is too little.
Android and Palm’s WebOS have a different but related problem. Instead of providing a home button, they provide a “back” gesture/button in addition to a home button. At first this appears to be better with its strong allusion to the ubiquitous browsing metaphor. But back on the phone is unpredictable: it might mean return to the last screen, the last area, or even the home screen. You never know where back will take you. Worse, there is no undo to “back”; without “forward” back becomes a minefield of maybes and didn’t means.
Another subtle problem of “back” is that it adds cognitive overload: you have to choose which to use, back or home. Because the functionality of home and back overlap this adds a non-insignificant Hick’s law penalty. It makes it hard to form a lasting habit, and lasting habits are the hallmark of a good interface.
The home button is too simple, and the back button’s mental model is too complex. What’s a better solution? We need a solution which is as simple and iconic as the iPhone’s home button, but provides a richer range of expression without the complexity of the Android/Palm back mechanism.