This Is Phil Schiller’s Grand Unified Theory of Apple by @FortuneMagazine December 4, 2015, 3:00 PM EST E-mail Tweet Facebook Linkedin Share icons Pressed earlier this year to explain why the world needs another Mac—or any desktop computer, for that matter—Apple’s senior vice president for worldwide product marketing had a ready answer. As a rule, Philip Schiller told Backchannel’s Steven Levy, you should be using the smallest possible device to do as much work as possible, before going to the next largest gadget in line. He then delivered what Levy described as Schiller’s “grand philosophical theory” of the Apple AAPL product line: “The job of the watch is to do more and more things on your wrist so that you don’t need to pick up your phone as often.” “The job of the phone is to do more and more things such that maybe you don’t need your iPad, and it should be always trying and striving to do that.” “The job of the iPad should be to be so powerful and capable that you never need a notebook. Like, why do I need a notebook? I can add a keyboard! I can do all these things!”” “The job of the notebook is to make it so you never need a desktop, right? It’s been doing this for a decade.” “[The job of the Mac] is to challenge what we think a computer can do, and do things that no computer has ever done before—[it should] be more and more powerful and capable so that we need a desktop because of its capabilities.” Students of Clayton Christensen’s The Innovator’s Dilemma—one of Steve Jobs’ favorite books—will recognize Schiller’s debt to what Christensen called the “job-to-be-done” theory of product development. Good product marketing, Christensen taught, starts with the job, not the customer. Here’s how he put it in a 2006 essay for the Harvard Business Review: “With few exceptions, every job people need or want to do has a social, a functional, and an emotional dimension. If marketers understand each of these dimensions, then they can design a product that’s precisely targeted to the job… The job, not the customer, is the fundamental unit of analysis for a marketer who hopes to develop products that customers will buy.” In other words, start with what people hire a gadget to do, rather than what the technology makes possible. “We have thought long and deep about what choices we want to offer customers,” Schiller tells Levy. “They are all computers. Each one is offering… something unique and each is made with a simple form that is pretty eternal.” Thanks to Steven Levy for scoring the Schiller interview. And to Above Avalon‘s Neil Cybart for bringing it to my attention. See also: If you only listen to one Apple podcast today… Apple’s feud with Samsung has been around almost as long as its product lines. This Fortune video is the latest update on that front: Follow Philip Elmer-DeWitt on Twitter at @philiped. Read his Apple coverage at fortune.com/ped or subscribe via his RSS feed. You can also subscribe to Data Sheet, Fortune’s daily newsletter on the business of technology.