雷瑞提出了第IV條件以處理葛棣爾問題對傳統知識三條件說所提出的難題，藉由指出在認知者接受系統中作為證成之證據的錯誤，認知者在證成遊戲中所構作的證成被擊敗了，雷瑞的理論解決了葛棣爾所提出的反例。基本上，雷瑞根據在認知者接受系統中所發現被證成使用的錯誤接受，而必須進行取代或替換的修正方式，解釋了認知者的知識歸屬情況。 批評者提出了葛棣爾類型反例質疑雷瑞所提之「可擊敗性條件」的充分性，在認知者接受系統中作為證成之證據沒有錯誤的情況下，批評者認為依據雷瑞的知識理論，認知者的情況得不到合理的解釋。本文即在處理諸位批評者所提出的論點與雷瑞所給予的回應之間的分析與比較，最後再針對此論題提出評論。

To solve the Gettier problem, Lehrer provides the defeasibility condition. The claimant appeals to anything false (false acceptance) in a justification game move, the skeptic is allowed to disqualify the move by the claimant. By this way, the claimant's justification is defeated. She lost the game to skeptic, and didn't know what she presented. Gettier's basic contention is that justified true belief may fall short of knowledge. Maybe the claimant has some justified true belief by luck. When the claimant depends on a false acceptance, the skeptic can ask her delete or replace it in her acceptance system. By Lehrer's theory, the claimant's justification is defeated. Lehrer explains the situation of knowledge attribution. Is the defeasibility condition a sufficient condition of knowledge? The critics give some Gettier-type counterexamples. In the examples of that kind, the claimant does need hot to base on the acceptance of false claim. Can Lehrer's theory explain those cases reasonably? In this article, I try to compare Lehrer's theory with his critic's, and give some comments in the end.