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.