“How long should it take for an app icon to animate up from its place on the home screen to fill the entire display? How far should you have to drag your finger on the screen for it to be possible to interpret the touch as a swipe gesture? How much should a two-finger pinch gesture allow you to zoom in on an image in the Photos app? The answers to all of these questions were numbers, and might be 0.35 seconds for the app animation, or 30 pixels for the swipe gesture, or 4x for photo zooming, but the number was never the point. The values themselves weren’t provably better in any engineering sense. Rather, the numbers represented sensible defaults, or pleasing effects, or a way to give people what they meant rather than what they did. It takes effort to find what these things are, which is appropriate, since the etymological root of “heuristic” is eureka, which (of course) comes from the Greek and means “to find.” This is where that word, “eureka,” actually figured into our development process, since good heuristics don’t come in brilliant flashes, but only after patient searches, and it wasn’t always clear to us that we had found the right heuristic even when we had. We arrived at our final decisions only with judgment and time. Heuristics are like this. They’re subjective.”