The ultimate goal of kendo is to achieve a state of mushin (empty mind), which means to react instinctivly to your oppponent without conciously thinking about it.
There are four sicknesses or shikai, 四戒, that prevent us from achieving this state of mind.
Shikai can be thought of as states of being that we must try to prepare ourselves against. The four sicknesses are:
1) Surprise (kyou)
2) Fear (Ku)
3) Doubt (Gi)
4) Confusion (Waku)
Is a symptom of an unprepared mind. It arises from circumstances that you were not expecting, you are taken unaware so much so that movement of the mind and body becomes erratic and unsynchronised. This leads to you being unable to make the right decisions or to take relevant action, resulting in an inability to respond in time.
Is a natural response to an oppressive force or to the unknown. Examples are; being scared of losing a point or getting hit by your opponent. It causes hesitation and or loss of initiative causing you to freeze and or not committing 100% to a cut or movement.
Doubting your own ability. You need to have confidence to achieve a state of mushin (you do things without actively thinking about it). Overthinking a specific shikaki-waza (attacking technique) or oji-waza (defensive technique) because you are unsure of your ability to execute will almost certainly result in a failed attempt, you have to re-act to these impulses naturally and with confidence. If you have doubt – you have already lost the battle.
Ties in strongly with doubt. When you have doubt, it leads to confusion. You become indecisive and lose the ability to make fast decisions or swift movements. You can’t respond to your opponent if you are confused about what you need to do.