Knowing one knows nothing, is sane; not knowing but claim one knows, is sickness.

-> The limited self, with its limited perception and bias, is not capable to see the whole picture in this unlimited universe.  Having some understanding and opinions and proceed to claim that one knows, is arrogance out of ignorance.

When one knows sickness is sickness, one will not be sick.

-> On the other hand, when one become honest and admit one’s limitation and bias, i.e. the self is limited and cannot know the universal wisdom, one is integrous and return to sanity.  I.e. a limited but sane person.

Saints are not sick, as they see sickness as sickness, and therefore not sick.

-> By admitting our limitations, we become true to what we are, and therefore do not fall into the temptation of self inflation and ignorance.  Saints are no difference, they do not claim their “self”s to be perfect and all knowing, but attribute that divine perfection to the universe and the Dao.

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Chapter 71 Discussion:

If claiming to know is sick, then are we all sick?  Yes, if we are unaware; and we become sane again when we become aware of it.

If our knowing is temporary and limited, how then should we view the knowing of the self then?  Should we stop trying to understand and know all together and claim that all knowledge and knowing are limited?  No, because if we do that we fall into another extreme of concluding all understanding as temporary.  Laozi said that, knowing the limitation of the self is adequate.  With this intuitive understanding, one can continue life with the self, in the natural rhythm of the universe, yet not fall into the sickness of self-inflation and ignorance.  Sanity returns.

Bilingual version of Ch71: https://www.lisiming.net/chinese-philosophy/daoist/daoist-philosophy/dao-de-jing-core-33-chapters/ddj55-79/