The hardest single part of building a software system is deciding precisely what to build. . . .

Therefore, the most important function that the software builder performs for the client is the iterative extraction and refinement of the product requirements. For the truth is, the client does not know what he wants. The client usually does not know what questions must be answered, and he has almost never thought of the problem in the detail necessary for specification. . . . Complex software systems are, moreover, things that act, that move, that work. The dynamics of that action are hard to imagine. So in planning any software-design activity, it is necessary to allow for an extensive iteration between the client and the designer as part of the system definition.

I would go a step further and assert that it is really impossible for a client, even working with a software engineer, to specify completely, precisely, and correctly the exact requirements of a modern software product before trying some versions of the product.

 

Much of present-day software-acquisition procedure rests upon the assumption that one can specify a satisfactory system in advance, get bids for its construction, have it built, and install it. I think this assumption is fundamentally wrong, and that many software-acquisition problems spring from that fallacy. Hence, they cannot be fixed without fundamental revision—revision that provides for iterative development and specification of prototypes and products.

 

I have seen most dramatic results since I began urging [incremental development] on the project builders in my Software Engineering Laboratory class. Nothing in the past decade has so radically changed my own practice, or its effectiveness. . . .

The morale effects are startling. Enthusiasm jumps when there is a running system, even a simple one. Efforts redouble when the first picture from a new graphics software system appears on the screen, even if it is only a rectangle. One always has, at every stage in the process, a working system.

— “No Silver Bullet: Essence and Accidents of Software Engineering,” Computer, Vol. 20, No. 4 (April 1987) pp. 10-19.