The two principal models of design in methodological circles in architecture—analysis/synthesis and conjecture/analysis—have their roots in philosophy of science, in different conceptions of scientific method. This paper explores the philosophical origins of these models and the reasons for rejecting analysis/synthesis in favour of conjecture/analysis, the latter being derived from Karl Popper’s view of scientific method. I discuss a fundamental problem with Popper’s view, however, and indicate a framework for conjecture/analysis to avoid this problem.