Standards are different for all things, so the standard set by man is by no means the only 'certain' standard. If you mistake what is relative for something certain, you have strayed far from the ultimate truth.

I know the joy of fishes in the river through my own joy, as I go walking along the same river.

The perfect man uses his mind as a mirror. It grasps nothing. It regrets nothing. It receives but does not keep.

The greatest tragedy that can befall a person is the atrophy of his mind.

There is a beginning. There is no beginning of that beginning ... There is something. There is nothing.

...the ten thousand things belong to one storehouse and life and death share the same body.

That knowledge which stops at what it does not know, is the highest knowledge.