One approach to development is based on some articulation of the requirements and then breaking them up to support building and deployment of elements of the system in an iterative manner but the integration is still left to the end. This means that you validate the assumptions only at the end and this is where the surprises are.

It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so - Mark Twain

An alternative is where we seek to integrate first and then build and deploy in an incremental manner with the requirements broken down to drive the development cycles. This means that any assumptions about the system as a whole can be explored early on.

“We never argue from facts to theories, unless by way of refutation or ‘falsification’. This view of science may be described as selective, as Darwinian. By contrast, theories of method which assert that we proceed by induction, or which stress verification (rather than falsification) … are typically Lamarckian: they stress instruction by the environment rather than selection by the environment.” - Karl Popper

So which is agile?