From Test Driven Development: By Example

The goal is clean code that works (thanks to Ron Jeffries for this pithy summary). Clean code that works is out of the reach of even the best programmers some of the time, and out of the reach of most programmers (like me) most of the time. Divide and conquer, baby. First we’ll solve the “that works” part of the problem. Then we’ll solve the “clean code” part. This is the opposite of architecture-driven development, where you solve “clean code” first, then scramble around trying to integrate into the design the things you learn as you solve the “that works” problem.