Mind the gap: in praise of informal sources of formal theory.

Theory-making can be taught and learned. New theories aim to fill a gap in existing explanations. Although mathematical statements are privileged elsewhere, social psychology's impactful theories tend to be verbal, perhaps because of the arbitrary scales of our variables. Good theories posit causal relations, attempt coherence, form a good narrative, aim for parsimony, are testable, prove fertile, and solve problems. Theories' sources can be intellectual, personal, group, or worldview. As long as the theory is stated logically, any source can be heuristic.