Fundamental problems in defining requirements
The fundamental problems that arise during system specification are [Keller 1989]:
• The goal/means conflict
• The determination and description of functional requirements
• The representation of the user interfaces

The goal/means conflict in system specification.
• The primary task of the specification process is to establish the goal of system development rather than to describe the means for achieving the goal.
• The requirements definition describes what a system must do, but not how the individual functions are to be realized.

Determining and describing the functional requirements.
• Describing functional requirements in the form of text is extremely difficult and leads to very lengthy specifications.
• A system model on the user interface level serving as an executable prototype supports the exploration of functional, nonfunctional and interaction-related requirements.
• It simplifies the determination of dependencies between system functions and abbreviates the requirements definition.
• A prototype that represents the most important functional aspects of a software system represents this system significantly better than a verbal description could do.

Designing the user interfaces.
• User interfaces represent a user-oriented abstraction of the functionality of a system.
• The graphical design of screen layouts requires particular effort and only affects one aspect of the user interface, its appearance.
• The much more important aspect, the dynamics behind a user interface, can hardly be depicted in purely verbal specifications.
Therefore the user interface components of the requirements definition should always be realized as an executable prototype.