Kevlin Henney's slides from his Keynote Talk at PHPNW09 conference. See http://phpcodemonkey.blip.tv for the associated video
Not sure about something? And that something affects the detailed design, an architectural decision or choice of functionality? Does that feel like a problem or a part of the solution?
There is a strong tendency for humans to feel unsure about uncertainty, in two minds over ambiguity and a little wobbly with instability. Whether over technology choice, implementation options, requirements or schedule, uncertainty is normally seen as something you must either suppress or avoid. Of this many people appear, well,
certain. That you should embrace it and use it to influence schedule, identify risk and inform design is not immediately obvious. A lack of certainty offers the opportunity to highlight risk and reframe questions, making uncertainty part of the solution rather than necessarily a problem.