冯契认为, 一切概念都有摹写现实与规范现实的双重作用, 逻辑原则也体现着后验性与先验性的统一。在逻辑理论的证成问题上, “概念的双重作用”要求逻辑理论的建构必须坚持后验性与先验性的统一。有见于现代逻辑论证理论的普遍主义、先验主义倾向, 非形式逻辑试图从后验性与先验性、普遍性与情境性的统一中去证成自身。

FENG Qi claims that any concept has a dual function of describing and prescribing reality, and that those basic logical rules embody a unity of a posteriority and a priority.Considering the problem of the justification of logical theory, FENG's such a theory requires that the construction of a logical theory adhere to the unity of a posteriority and a priority.The justification of the argument theory in modern logic takes a more universalistic and a prior approach; however, informal logic tries to justify itself from the perspective of a unity of a posteriority and a priority, and of universality and contextuality.