68. But if we attribute one substance to the Father and the Son to teach that there is a solitary personal existence although denoted by two titles: then though we confess the Son with our lips we do not keep Him in our hearts, since in confessing one substance we then really say that the Father and the Son constitute one undifferentiated Person. Nay, there immediately arises an opportunity for the erroneous belief that the Father is divided, and that He cut off a portion of Himself to be His Son. That is what the heretics mean when they say the substance is one: and the terminology of our good confession so gratifies them that it aids heresy when the word o0moou/sioj is left by itself, undefined and ambiguous. There is also a third error. When the Father and the Son are said to be of one substance this is thought to imply a prior substance, which the two equal Persons both possess. Consequently the word implies three things, one original substance and two Persons, who are as it were fellow-heirs of this one substance. For as two fellow-heirs are two, and the heritage of which they are fellow-heirs is anterior to them, so the two equal Persons might appear to be sharers in one anterior substance. The assertion of the one substance of the Father and the Son signifies either that there is one Person who has two titles, or one divided substance that has made two imperfect substances, or that there is a third prior substance which has been usurped and assumed by two and which is called one because it was one before it was severed into two. Where then is there room for the Son's birth? Where is the Father or the Son, if these names are explained not by the birth of the divine nature but a severing or sharing of one anterior substance?