The interface IS the software

I wrote yesterday about technical myopia, and I wanted to point out how the 37 signals team gets their priorities right. Here's a quote from their book "Getting Real":

Getting Real starts with the interface, the real screens that people are going to use. It begins with what the customer actually experiences and builds backwards from there.This lets you get the interface right before you get the software wrong.

Why? Because to the lay user, the interface IS the software. If something isn't in the interface, it doesn't exist.