From Proclus, "On providence and fate and what depends on us: A reply to Theodore the engineer," §39 (trans. Steel):
"In some cases we say that the celebrated mantic and the rituals of the priests have a place in whether something happens or not, in other cases we say that they are dominated by what comes forth from the universe. Thus, when all the divine and demonic causes in the universe contribute to one effect and speak as it were with one voice, then also the theurgical works seem to be ineffective [in altering the outcome] … But whenever, in a situation where there are many agents, some bring forth one thing and some another, for instance, some bring forth corruption and others a cure for those who are ill … then the application of theurgy may dissolve the influxes that come down from harmful agents, using as co-operative powers the influxes of the agents that are beneficial to us …"
"In some cases we say that the celebrated mantic and the rituals of the priests have a place in whether something happens or not, in other cases we say that they are dominated by what comes forth from the universe. Thus, when all the divine and demonic causes in the universe contribute to one effect and speak as it were with one voice, then also the theurgical works seem to be ineffective [in altering the outcome] … But whenever, in a situation where there are many agents, some bring forth one thing and some another, for instance, some bring forth corruption and others a cure for those who are ill … then the application of theurgy may dissolve the influxes that come down from harmful agents, using as co-operative powers the influxes of the agents that are beneficial to us …"