But such a religion could not satisfy ardent and logical minds, like those of St. Hilary and his two great successors, St. Ambrose and St. Augustine. To such men it was a necessity of their faith that they should know, and know in its right proportions, the truth so far as it had been revealed, and trace the appointed limits which human knowledge might not overpass. For their own assurance and for effective warfare against heresy a reasoned system of theology was necessary. Hilary, the earliest, had the greatest difficulty. To aid him in the interpretation of Scripture he had only one writer in his own tongue, Tertullian, whose teaching, in the matters which interested Hilary, though orthodox, was behind the times. His strong insistence upon the subordination of the Son to the Father, due to the same danger which still, in the fourth century, seemed in the East the most formidable, was not in harmony with the prevalent thought of the West. Thus Hilary, in his search for reasons for the Faith, was practically isolated; there was little at home which could help him to construct his system. To an intellect so self-reliant as his this may have been no great trial. Scrupulous though he was in confining his speculations within the bounds of inherited and acknowledged truth, yet in matters still undecided he exercised a singularly free judgment, now advancing beyond, now lingering behind, the usual belief of his contemporaries. In following out his thoughts, loyally yet independently, he was conscious that he was breaking what was new ground to his older fellow-Christians, almost as much as to himself, the convert from Paganism. And that he was aware of the novelty is evident from the sparing use which he makes of that stock argument of the old controversialists, the newness of heresy. He uses it, e.g., in Trin. ii. 4, and uses it with effect; but it is far less prominent in him than in others.