This one is found quite often on Facebook, on blogs, etc.
It certainly doesn’t ring true to me. While clear thinking is a useful and necessary quality to cultivate, ultimately the Dharma (or truth) is beyond thought.
I suspect this quote is a poor paraphrase of one of two expressions.
One is a formula known as the “Recollection of the Dhamma” (dhammanusati), which is encapsulated in a stock phrase that’s found many times in the Pali scriptures. It runs like this in a more normal translation:
“The Dhamma is well-expounded by the Blessed One, to be seen here and now, timeless, inviting verification, pertinent, to be realized by the wise for themselves (veditabbo viññūhī).”
The other is a separate although similar expression of the qualities of the Dhamma:
“Deep … is this Dhamma, hard to see, hard to realize, tranquil, refined, beyond the scope of conjecture, subtle, to-be-experienced by the wise (paṇḍitavedanīyo).”
No matter how clearly we think about the Dhamma, it’s ultimately not thinking that brings the kind of wisdom the Buddha is talking about here. As the second quote above says, the Dhamma is “beyond the scope of conjecture (atakkāvacaro)” — a phrase which has also be translated as “unattainable by reasoning” or “being beyond the sphere of thought.”
The Dhamma is something “to be experienced.” It’s something to be seen or realized (“realized” here not in the sense of “understood intellectually” but in the sense of “having made real in an experiential sense.”)
So far I’ve seen this one in only one book: “Buddhism In North-East India,” by Sristidhar Dutta and Byomakesh Tripathy. There it’s found in the even more unlikely form; “The dharma that I preach can be understood only by those who know how to think and intellectual people who have the intelligence to use their minds clearly and know how to appreciate this dharma as a universal law.”
This quote, even more than the more common shorter version, may well be part of an attempt to make Buddhism seem more “rational” and therefore more palatable to modern readers.