The paper proposes a critical1 approach to information systems definition, design, and development (ISD) grounded in discourse theory, semiotics, practical philosophy and critical systems thinking. It aims to support IS researchers and practitioners in the difficult process of identifying and scrutinizing the diverse issues they face in any ISD project. Two main components of the approach are a hierarchic arrangement of these issues, and of the basic kinds of validity claims they imply, in the form of a philosophical staircase; and a practical framework for critical discourse on these claims called critically systemic discourse. The present first part introduces the staircase and discusses the relevance of the discursive principle for dealing with the various validity claims raised by each one of its steps. The second part will present a discursive framework for applying the staircase
