This paper will present a means of justifying knowledge claims (research findings, conclusions) about technology-assisted social systems. It uses the ideas developed by the systems thinker C. West Churchman (1914–2004), so individual experience and experimentation are seen as important, but insufficient for dealing with the design of social systems. Richer criteria for justification are required, including consideration of stakeholders' perspectives and alternative explanations. This paper will explore the conjecture that Churchman's five "guarantors' of knowledge can provide systems designers and researchers with criteria for justifying their findings, their claims to know something. The evidence to support this argument will be drawn from Churchman's books, 'systems Thinking and Its Enemies' and "The Design Of Inquiring Systems' as well as the argumentative inquiry and perspectival thinking literature. A list of "justification" criteria for researchers is provided as a summary.