I came across this sentence from Cicero in my revision of conditional sentences:
non potest iucunde vivi nisi cum virtute vivatur = it is impossible to live happily except by living virtuously.
Why is vivi (the present passive infinitive) used, and not vivere (the active infinitive)?
I can’t seem to find anything in any grammar books on this. I would be grateful if someone could explain it to me.
Thank you in advance!
Potest is used impersonally when joined with an impersonal verb. For other examples see here.
Thank you! So does it literally mean ‘it is not able to be lived happily, unless it is lived virtuously’? 
Maybe you can thing that “it” is actually “life”.
Thanks
Latin expresses impersonal by passive, sometimes at least:-
it = he goes; itur = one goes, a journey is made
vivit = he lives; vivitur = one lives, life goes on
Thank you