(France is bacon)
(Taigi wa taigo no motoi;
“Great doubts are the foundation of great enlightenment”)
Great doubt is the first step toward great realizations. Doubt leads to asking questions, which leads to seeking answers, which – in theory at least – eventually leads to finding answers that leave you with a broader and deeper understanding.
We begin with the compound noun 大疑 (taigi), “big doubt,” marked by the particle は (wa) as the topic of discussion. The comment on this topic is another noun phrase; compound noun 大悟 (taigo), “big revelation,” marked by the associative particle の (no) as attaching to and possessing the noun 基 (motoi), “foundation.” One may imagine an elided copula to make the phrase into a complete sentence.
Keep in mind that 悟 isn’t just any random realization; it refers specifically to Buddhist enlightenment about the true nature of the universe.
基 may also be read as moto without any change in meaning; my sources seem evenly split on which is considered the “main” reading and which the alternate. Variant phrasings may make the same general point with a double negative, such as 疑わぬ者に悟りなし (utagawanu mono ni satori nashi), “for those who do not doubt, there is no satori.” Another variant takes this form but replaces 疑わぬ with 迷わぬ (mayowanu), i.e. “getting lost” instead of “doubting.”
One possibly-interesting footnote is that, since for my current いろは-ordered series I’ve been trying to use phrases from the major iroha karuta sets, my original plan was to do a write-up for a saying that goes 大食上戸の餅食らい (taijiki jouto no mochi-kurai). (This is the た entry in the Osaka/Nagoya set; I’ve already used the entries for the Edo and Kyoto sets.)
The thing is, there doesn’t seem to be much reliable information on this phrase online. None of my regular sources include it, and a further search only pointed me to a handful of rambling blog posts that give opinions without citing authoritative sources. There’s a chance that a trip to the East Asian collection at the local university library would provide something more solid (they have several books on kotowaza), but in the end it was simply more efficient to choose a better-documented phrase.
(“Taigi wa taigo no motoi da to senmonka no shuchou wo kawasu koto wa kesshite warui koto de wa nai kedo, jibun no kangae ni tai suru gimon mo motsu beki da.”)
[“It’s not necessarily a bad thing to say that ‘great doubt is the basis of great understanding’ to deflect what experts have declared, but it is necessary to entertain questions about your own thinking.”]