When trying to understand how designers work, it is tempting to examine design products in order to come up with the principles or norms behind them. The problem with such an approach is that it may lead to a purely syntactical analysis of design artifacts, failing to capture the knowledge of the designer in an explicit way, and ignoring the interaction between the designer and the evolving design. We will present a theory about design activity based on the observation that knowledge is brought into play during a design task by a process of interpretation of the design document. By treating an evolving design in terms of the meanings and rules proper to a given way of seeing, a designer can reduce the complexity of a task by focusing on certain of its aspects, and can manipulate abstract elements in a meaningful way.