"The power of instruction is seldom of much efficacy, except in those happy dispositions where it is almost superfluous."

Edit: from here, the context:

[link|http://www.math.columbia.edu/~pinkham/MVD_Acceptance_Speech.html|http://www.math.colu...tance_Speech.html]

As Gibbon says, "the monstrous vices of the son" - Commodus - "have cast a shade on the purity of the father's motives", because, " he sacrificed the happiness of millions to a fond partiality for a worthless boy" in making Commodus his successor. The vices of Commodus are described at length by Gibbon, and I refer you to chapter IV of the Decline and Fall for details. Gibbon goes on to say:

Nothing was neglected by the anxious father, and by the men of virtue and learning whom he summoned to his assistance, to expand the narrow mind of young Commodus, to correct his growing vices, and to render him worthy of the throne. But the power of instruction is seldom of much efficacy except in those happy dispositions where it is almost superfluous. The distasteful lesson of a grave philosopher was, in a moment, obliterated by the whisper of a profligate favorite; and Marcus himself blasted the fruits of this labored education, by admitting his son, at the age of fourteen or fifteen, to a full participation of the Imperial power.