यत्तु कृत्स्नवदेकस्मिन्कार्ये सक्तमहैतुकम्।अतत्त्वार्थवदल्पं च तत्तामसमुदाहृतम्॥
yat tu kṛtsna-vad ekasmin kārye saktam ahaitukamatattvārtha-vad alpaṃ ca tat tāmasam udāhṛtam
Word by word
The verse says
And the knowledge that clings to one single thing as though it were the whole, with no reason behind it, missing what is actually the case, and narrow: that is called dark.
In plain words
The worst kind, and four things are said about it.
It takes one piece for the whole. It has no reason behind it. It is not in contact with how things are. And it is small.
That third phrase is the sharp one. This knowledge is not simply incomplete. It has lost touch with the thing it is supposedly about.
Where you've felt this · in failure
Your neighbour had one bad experience at a hospital twenty years ago. He decided then that doctors cannot be trusted. He has had no other evidence since, because he has not been back. The conclusion has not been revised in twenty years, for the simple reason that nothing has been allowed near it.
Going deeper
A single fragment held as the whole truth, sealed off from testing, is the condition described, and its defining feature is that it cannot be updated.
The four marks go together. If you take one piece for the whole, you no longer need any other evidence. If you have no reason behind the belief, no reason can dislodge it. The narrowness is not incidental; it is what keeps the structure standing.
This is the knowledge equivalent of what Chapter 16 called stiffness: not an arguer, but someone the question never reaches. Nothing is being defended, because nothing has been examined.
And the honest application is inward, since everybody can name someone else who does this. The question worth sitting with is which of your own settled conclusions was last tested, and whether you have arranged your life so that it never has to be.