「侔」式推論的肯定式「是而然」是公認的有效推論。根據周云之先生，它的有效性有兩個意義：一、它是演繹有效的，二、它是辭義齊等的。本文企嘗試論證的是，如果我們接受了「是而然」推論形式其有演繹的有設性，就可能使它失去辭義齊等的意義。

The 'inference from affirmative to affirmative (是而然)', the affirmative form of type 'Mo(侔)' reasoning , is generally accepted as valid. Accordingly to Mr. Zhuo Yun Zhi, its validity has two meanings: firstly, it is deductively valid; secondly, there is an equivalence between meanings of premise and conclusion (辭義齊等). This article argues if the validity of 「是而然」 is accepted as deductively valid, then the other meaning of validity (辭義齊等) will be lost.