In this presentation filmed during Agile 2008, Ron Jeffries and Chet Hendrickson derive Agile practices from the natural laws of software development. They don't just say "Be Agile!", but they explain why Agile practices make perfect sense in the software development world.
Watch: Natural Laws of Software Development (1h 30 min.)
Ron and Chet say that, usually, in business, nothing goes according to the plan, but in all other businesses, except software development, they know before hand they are not going to meet the deadline, and proceed accordingly. We need to provide the same ability to have predictability. It is not possible to have predictability if one is given all the specifications and attempts to complete an entire project before a deadline is met because nobody knows for sure how much of the project has been done. Instead, if the project is approached step by step by doing chucks of it, one could figure out if the project is in graphic or not.
Ron and Chet continue by saying that if we deliver Running Tested Features, then the business management can have predictability, early releases, and will request high value features first, which in end will be a return of investment. Businesses need incremental software development: "if they have it, they can manage it, make more money with it, understand what's going on with it."
Defects may be accounted as negative features. Not knowing what they are means not knowing for certain where we are in the development process. An important cause of defects is misunderstanding the requirements, which happens easily when a team has to deal with all of them at once, instead of incrementally receiving and addressing them. Requirements are well understood during conversations person to person. User stories come handy because they open the conversation which conducts to confirmation. The end result is clarity, the clear understanding of requirements.
In that manner, Ron and Chet derive in their presentation all Agile practices as inherent effects of the natural laws of software development.