知不知上；不知知病。
1. To know and yet (think) we do not know is the highest (attainment); not to know (and yet think) we do know is a disease.
夫唯病病是以不病。聖人不病。以其病病，是以不病。
2. It is simply by being pained at (the thought of) having this disease that we are preserved from it. The sage has not the disease. He knows the pain that would be inseparable from it, and therefore he does not have it.
Legge's Comments
知病, 'The Disease of Knowing.' Here, again, we have the Dào working 'by contraries,'—in the matter of knowledge. Compare par. 1 with Confucius's account of what knowledge is in the Analects, II, ch. 17. The par. 1 is found in one place in Huáinán, lengthened by the addition of particles; but the variation is unimportant. In another place, however, he seems to have had the correct text before him.
Par. 2 is in Hán Fēi also lengthened out, but with an important variation (不病 for 病病), and I cannot construe his text. His 不 is probably a transcriber's error.