Mohist Optics and Analogical Reasoning

In Mohist philosophy, the gnomon is a metaphor for the standard of valid arguments. This metaphor comes from the method of establishing due east and west by observing gnomon shadows at dusk and dawn. I argue that there is also an overlooked, implicit aspect of the gnomon metaphor that comes from its function of measuring the height of heaven indirectly through proportional calculation. The function of indirect measurement inspires a strategy of argumentation in Mohist ethics, which I call “analogical upscaling.” Such an argument draws a parallel between small-scale and large-scale actions for the purpose of determining the Will of Heaven in a particular circumstance, just as the gnomon measures heaven’s physical height analogically. In the conclusion, I compare the epistemological status of light and shadow in the Mozi and Plato’s Republic.