1. By transmitting to thee, my very dear friend, this fourth book of the work which is [entitled] The Detection and Refuation of False Knowledge, I shall, as I have promised, add weight, by means of the words of the Lord, to what I have already advanced; so that thou also, as thou hast requested, mayest obtain from me the means of confuting all the heretics everywhere, and not permit them, beaten back at all points, to launch out further into the deep of error, nor to be drowned in the sea of ignorance; but that thou, turning them into the haven of the truth, mayest cause them to attain their salvation.
2. The man, however, who would undertake their conversion, must possess an accurate knowledge of their systems or schemes of doctrine. For it is impossible for any one to heal the sick, if he has no knowledge of the disease of the patients. This was the reason that my predecessors–much superior men to myself, too–were unable, notwithstanding, to refute the Valentinians satisfactorily, because they were ignorant of these men’s system;(1) which I have with all care delivered to thee in the first book in which I have also shown that their doctrine is a recapitulation of all the heretics. For which reason also, in the second, we have had, as in a mirror, a sight of their entire discomfiture. For they who oppose these men (the Valentinians) by the right method, do [thereby] oppose all who are of an evil mind; and they who overthrow them, do in fact overthrow every kind of heresy.