Teaching and promoting conceptual thinking in engineering education is challenging. Two recently introduced design thinking models are compared in order to explore the reasons behind this challenge. The comparison results in key distinctions between divergent and convergent thinking, and between concept and knowledge domains. The differentiating principle is shown to be a common principle of the two models, truth-value, or logical status, of the propositions that engineers make. Building on this insight, divergent thinking by inquiry is identified as a mechanism for promoting conceptual thinking, and a specific implementation of engineering portfolios is proposed as a pedagogical tool.