We used to design for the web. At first, this was just electronic print, but we realized it should be interactive; then personalized. Today, more people access the world through mobile devices. Designers are trying to teach clients that you can’t mouse-over or right-click on a tablet—and you can’t swipe well on a computer.

“Mobile first!” they cry.

Not so fast. Mobile devices aren’t consumption devices, they’re prosthetic brains. And that means they need to tell us when they have useful information (interruption); and answer our questions (contextual search).

This is the trend we see with Siri, as well as Google Now and Field Trip. Mobile design is partly smart memory (which knows, for example, that when you say “where’s a good sushi restaurant?” at 4PM it should find something nearby, perhaps that your friends have recommended, and within your price range.) And it’s partly smart interruption (which says, “you should leave now or you’ll miss your meeting” because it knows where you are, what’s on your calendar, and what traffic is like along your most likely route.”)

Techstars’ Brad Feld nailed this in a March, 2010 post entitled Email is still the best login, and Fred Wilson calls email social media’s secret weapon. Because email jumps into your lap. It interrupts you. Want to know why SMS is still popular? Because unlike many other messaging platforms, it always pushes itself onto your screen, vaulting past the gates of opt-out and commanding your attention.

But do it too often, or for unimportant reasons, and you’re the app that cried wolf. You need to be clever, knowing when to interrupt someone. After all, a spare brain that simply distracts your main one is a recipe for ADD. That’s why smart notification services like Prismatic use machine learning to mine your social graph and the things you discuss, sending you targeted reading lists.

Saying “mobile first” is wrong. Too often, that just means “make this web page work on a tablet.” Just as web designers who simply took offline documents and rendered them in HTML missed the point of the Web, so mobile designers that simply make the web work on a phone miss the point of mobility.

Mobile isn’t the point. Interruption is.

We don’t call interruption an interface, really. It’s almost an afterthought. Blackberry did it reasonably well with a small bar at the top of the screen; Android had a decent one from the start; Notification Center was slowly built into MacOS and IOS. But few companies design for “interruption first,” even though that’s how most of us engage with the services on which we’re most active.

Ask yourself: how often do you say, “I should go on Facebook”? And how often do you instead see “someone commented on your post” and start from there? The reality is that, in a mobile posture, we start by being interrupted. Services that interrupt well avoid disengagement, which is the worst thing that can happen to a startup.

Stop worrying about taps, screens, or swipes. And start worrying about how to interrupt your users well.

Sidenote: My co-author Ben Yoskovitz and I have been writing about this stuff, and how to measure it, in our forthcoming book, Lean Analytics. We’re learning a lot.

The book is part of O’Reilly’s early release program, which means you can buy it now and get updates as we finish it off. Right now we’re up to 150 pages. If you want to buy a copy, you can do so here.