#The case against re-running all tests every time you change a thing

From Rich Hickey's Simple Made Easy talk: "I can make changes 'cause I have tests! Who does that?! Who drives their car around banging against the guard rails saying, "Whoa! I'm glad I've got these guard rails!"

Tests are primarily a sanity check. We should have confidence in our code because we understand it and are able to reason about it.