The four sicknesses (surprise, fear, doubt, confusion)

Sagye (사계, 四戒) is a mental concept in kumdo. The glossary of the Korea Kumdo Association (대한검도회) defines it as gyeonggu-uihok (경구의혹, 驚懼疑惑), four disturbed states of mind. Another Korean source describes it as a warning against four wrongful states of mind (삿된 마음). English-language writers usually render it as the "four sicknesses".

• Gyeong (경, 驚), surprise: something unexpected happens and the mind is shaken. Mind and body are briefly thrown into confusion, so normal judgement and a suitable response are not possible.
• Gu (구, 懼), fear: dread, which dulls mental activity and makes the hands and feet lose their movement. One should not be afraid because the opponent is big, because their shout (기합, gihap) is loud, or because they put on a show of strength.
• Ui (의, 疑), doubt: with doubt one cannot judge with a normal mind and cannot reach a decision.
• Hok (혹, 惑), confusion: the mind wanders and the spirit stagnates, so quick judgement and light action become impossible.

If even one of the four is in the mind, the mind is thrown into disorder and the opponent's opening (틈, teum) cannot be seen. The practitioner also shrinks back and creates an opening of their own. Another Korean source adds that the spirit loses its unity and the opponent can strike, which is why kumdo teaches this as mental training.

The Association's glossary links the term to the ordinary mind (평상심, pyeongsangsim): what damages it is gyeonggu-uihok, that is, the sagye. It also says that in the state of no thoughts (무념무상, munyeom-musang), reached only after advanced training, the sagye are entirely absent.