Thomas Aquinas, Compendium of Theology

Hence we are instructed in the rule of Catholic faith to profess that the Son is "consubstantial with the Father," a phrase that excludes two errors. First, the Father and the Son may not be thought of according to carnal generation, which is effected by a certain separation of the son's substance from the father. If this were so in God, the Son could not be consubstantial with the Father. Secondly, we are taught not to think of the Father and the Son according to intellectual generation in the way that a word is conceived in our mind. For such a word comes to our intellect by a sort of accidental accretion, and does not exist with the existence proper to the essence of the intellect.