The principles of customer development were devised by Steve Blank way back in the 1990s. So why does every third startup continue to fail because of ignoring them? Customer development tends to be a painful process. It requires startup founders to shift their mindset. Instead of blindly believing that their ideas and gut feelings are rock-solid, they need to find gaps in their ideas, prove themselves wrong, and invalidate their hypotheses. Who wants to beat themselves up of their own will?