Abstract: Many Scholars who study “Moist Classics”define “xiao(效)”as a specific mode of demonstration or a principle of “shuo(说)(i.e.statement)”.Having analyzed the definitions stated by previous scholars,the author argues that “xiao”is neither a mode of demonstration on some specific form of deduction nor a condensation for the principle of “shuo”,but an important category which is closely related to the methodology of studying logic of “Moist Classics”.For “xiao”stipulates the object,character and purpose of the study of “Moist Classics”logic,and takes a high status in logic of “Moist Classics”.