Huan T’an  43 bce – 28ce
1— I told Liu Tzu-chün that cultivating one’s nature does not help (postpone old age).
2— His elder brother’s son Po-yü said “If Heaven produces drugs which kill man, it must certainly produce drugs which gives man life.”
3—I said, “The plant ourouparia rhynchophylla does not agree with man, so eating it causes death. But it is not produced for the purpose of killing man. Similarly, Szechwan beans poison fish, arsenic kills rats, cassis injures otters, and apricot stones kill dogs. But these are not made by Heaven for this purpose.
Fragment 66, p. 80-81.

Huan T’an (-43, +28) Hsin-lun (New Treatise) and other Writings.
An annotated translation with index by Timoteus Pokora. Michigan Paper in Chinese studies, The University of Michigan, Center for Chinese Studies. (our numbering

1 — The discussion focuses on the length of human life, the possibility of prolonging or even making it eternal.
« Cultivating one’s nature » is a key Confucean imperative for living a better life, but it is rejected here as a mean of extending life.

2 — Po-yü implicitly agrees with this negative conclusion and proposes another theoretical posibility, using the topos of the opposites:
Some plants cause death, so there must be plants which give life
The search is not about medicinal plants, known by the Chinese since the Divine Farmer told Human about them, but about plants giving eternal life. 

Since there are toxic plants, so there must be healing plants, The reasoning is, since the opposite  tems are mutually  defined (nice presupposes disagreeable, positive/negative, etc.). Speaking of toxic plants presupposes the existence of curative plants. 

3. Paragraph 3 refutes Po-yü’s belief in Heaven’s benevolent intentions toward humanity.
The issue is the teleological structure and providential organization of the world. Oranges are naturally divided into segments, but it It would be absurd to interpret this as a suggestion by Goddess Nature that the fruit should be shared between the members of the group.

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“If Heaven produces drugs which kill man, it must certainly produce drugs which gives man life.”