Problem-solving is widely recognised as a core cognitive process essential for navigating both routine daily activities and complex professional challenges. Its development is a fundamental learning outcome across diverse life contexts. Design is inherently a problem-identifying and problem-solving activity that hinges on the formation of assumptions to address problems. Assumptions, defined as judgements without conviction, exert specific limitations on problem-solving processes. Modifying initial assumptions can considerably alter the outcomes of a problem. Although researchers often explicitly state their assumptions, the origins of these assumptions are not comprehensively addressed. The paper explores concepts related to assumptions and cognition, emphasising the gap in existing literature due to the absence of a well-established theory of assumptions. Reviewing these concepts indicates that the assumption-making process necessitates empirical verification through behavioural experiments. Potential research questions have been outlined in the paper. Initial findings have been presented, which underscore variations in responses among different age groups. This research is a step forward in developing a comprehensive theory of assumptions that captures the dynamics of assumption-making within problem-solving.