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In Zen Buddhism, the greater your doubt, the greater will be your enlightenment. That is why doubt can be a good thing. If you are too sure, if you always have conviction, then you may be caught in your wrong perception for a long time.

Thich Nhat Hanh (2008). “Buddha Mind, Buddha Body: Easyread Comfort Edition”, p.72, ReadHowYouWant.com
In Zen Buddhism, the greater your doubt, the greater will be your enlightenment. That is why doubt can be a good thing. If you are too sure, if you always have conviction, then you may be caught in your wrong perception