Taigi-wa Taigo-no Moto (大疑は大悟の基)
Taigi (大疑・たいぎ): Great Doubt – having a big doubt
Taigo (大悟・たいご): Great Enlightenment – getting enlightened/realizing something very important
Moto (基・もと): Basis
Great doubt is the beginning of great enlightenment.
You are encouraged to have a big doubt about something that seems important. Keep working on that until what you have thought about becomes crystal clear to you. That is the moment when you are really enlightened to the next level.