This paper presents a systemic intervention in which a group of researchers and engineering students used critical systems thinking ideas to promote reflexivity on a basic concept that underlies engineering practice: efficiency. In particular we explored a situation in which students had to deal simultaneously with issues of efficiency and justice. Engineering students are frequently trained to design efficient systems or to improve the efficiency of already existing systems. Although engineering and economic efficiency are not the same, young engineering students tend to equate and value both of them. However, within many contexts efficiency is not the only relevant criteria for judging among different alternative solutions to engineering problems. Justice and other ethical considerations are also frequently relevant. Not all efficient technical solutions are also the most fair, and vice versa. In this paper we describe a research inquiry in which a group of engineering students were invited to choose among diverse solutions, involving issues of justice and efficiency. Based on the work of a group of scholars such as John Rawls, John Nash, Robert Aumann, and Howard Raiffa, the students explored different conceptions of justice as well as their relationships with efficiency. During this process that involved both individual and collective work, we found evidence that the students become engaged in uncovering and questioning their ways of thinking and behaving, as well as their moral frameworks. Initially we found a tendency among engineering students to be unwilling to deviate from the solutions that involve Pareto efficiency, to give priority to efficiency over justice, to understand justice only within the context of efficiency, as well as to experience difficulties in developing rational arguments to reach rationally justifiable conclusions on issues of justice and efficiency. The research revealed that senior undergraduate and master engineering students frequently experience a substantial difficulty in arguing coherently in debates about practical rationality, something that is in stark contrast with their good ability to deal with technical issues and mathematical calculations. At the beginning of the experiment disputed questions related to justice and efficiency were frequently treated not as a matter of rational enquiry and justification, but as a problem of personal opinions and unarticulated presuppositions that were relegated to the realm of irrational acts of belief. The use of boundary critique and several critical systems thinking tools contributed to change the way engineering students made and justify their choices among competing conceptions of justice, and to develop a new notion on how to reach a balance between efficiency and justice. The discussion helped students to reflect on wider issues that involved their role in issues of social justice in their society. The changes that students experienced were not the result of the researchers’ intentions to convince the students of making some particular choices, but of a dialogical rather than a monological approach to ethical issues and practical rationality. This dialogical approach involved the exploration of different alternative boundary judgements that promote reflexivity on what and whose views are included in or excluded from analysis. Students were able to understand that different ethical choices result from choosing diverse boundary judgements.