Like many of Apple’s inventions, the iPhone began not with a vision, but with a problem. By 2005, the iPod had eclipsed the Mac as Apple’s largest source of revenue, but the music player that rescued Apple from the brink now faced a looming threat: The cellphone. Everyone carried a phone, and if phone companies figured out a way to make playing music easy and fun, “that could render the iPod unnecessary,” Steve Jobs once warned Apple’s board, according to Walter Isaacson’s biography. Read the full story at Slate.