Even if a startup is doing user-centered design, or optimizing their product through split-testing, or conducting tons of surveys and usability tests, that’s no substitute for also doing customer development. It’s the difference between asking “how should we best solve this problem for these customers?” and “what problem should we be solving? and for which customer?” These two activities have to happen in parallel, forming a company-wide feedback loop. We call such companies built to learn. Their speed should be measured in validated learning about customers, not milestones, features, revenue, or even beautiful design. Again, not because those things aren’t important, but because their role in a startup is subservient to the company’s fundamental purpose: piercing the veil of extreme uncertainty that accompanies any disruptive innovation.