How to translate 乎 in classical Chinese texts? I know that 乎 usually expresses doubt and is sometimes translated using rhetorical questions. But is it always true? I am wondering how should it be translated in such a case: "而有以通乎性命之全". The full sentence is:  

惟人也，得其形氣之正，是以其心最靈，而有以通乎性命之全

For the sake of clarity - the sentence is preceded with another one that contains the following statement: "然体有偏正之殊".
is it always true


no. only when 乎 is at the end of a verse, then it might be interpreted as doubt, rhetorical question, or guess.

http://dict.revised.moe.edu.tw/cgi-bin/cbdic/gsweb.cgi?ccd=hj97ow&amp;o=e0&amp;sec=sec1&amp;op=v&amp;view=0-1

in the verse


  而有以通乎性命之全


"乎" means 於, a preposition (at, in, from, . . . etc).


  惟人也．得其形氣之正．是以其心最靈．而有以通乎性命之全


i would suggest to interpret the verse as:

only the homo sapiens (惟人也),

get (得) the upright (正) body (形) &amp; vital energy (氣),

therefore (是以), its (其) mind (心) is the most (最) divine; (靈)

and (而) depends on [such upright body and vital energy] (有以), [homo sapiens ] succeed (通) in (乎) the complete (全) life (性命)



have fun :)