Designers invariably use imagery to generate new form combinations which they represent through sketching. But they also do the reverse: they sketch to generate images of forms in their minds. Common belief regards such activity as non-rational. In contrast, we assert that interactive imagery through sketching is a rational mode of reasoning, characterized by systematic exchanges between conceptual and figural arguments. Cognitive science, strongly dominated by a linguistic paradigm, has yet to recognize the paramount role of visual reasoning in many instances of problem solving, and in design tool-making, computational and otherwise, we must learn to optimize rather than bypass intuitive visuality.