The type of feeling of confusion and distrut for your sense that some Koans attempt to instill.
Ciro Santilli's preferred version of it is physics and the illusion of life.
Some notable references:
This is perhaps the most popular version of Great doubt in 21st century China
Fuller quote as seen e.g. at: www.hrfjw.com/fjrw/hcrw/ywfs/324750.html
大疑大悟,小疑小悟,不疑不悟
translation;
Great doubt, great underestanding. Small doubt, small understanding. No doubt, no understanding.
k.sina.cn/article_5668613235_151e02c7300100wutq.html attributes it to modern day Chinese "Master Chongci" (崇慈法师).