St. Augustine and Pelagianism
Although Pelagianism promoted moral fervor, there was an inherent danger in it: self-reliance, not God-reliance, based upon an inadequate understanding of human nature. Pelagianism stressed complete human autonomy and freedom of the will before God. Pelagius posited three elements to any moral action: 1. that we must be able to do it, 2. that we must be willing to do it, and 3. that the action must be carried out. Or the three elements can be described as possibility, will, and action. Possibility is a natural gift from God alone, but the other two, since they arise from man's choice, are from man. For instance, God has freely given us the gifts of speech, sight, hearing, etc., and the power to speak, see hear, etc., yet whether or not these are put to good use is left entirely up to the individual. Thus, we are entirely free to will and do good or evil. Nor does he separate will from power, finding in the will the power to automatically carry out what it has willed.The piece is a gem. Read and enjoy!
Yet, as most of us are all too aware, the ability to do good is not merely a matter of willing it, "for I do not the good I will, but the evil I do not will, I perform" (Rm.7:19). Or, as Augustine puts it: "Whence comes this monstrous state? Mind commands body and it obeys forthwith. Mind gives orders to itself, and it is resisted . . . . Mind commands mind to will-but it does not do so. Whence comes this monstrous state? Why should it be? I say that it commands itself to will a thing: it would not give this command unless it willed it, and yet it does not do what it wills." 
Thus, while refuting Manicheanism--the theory of the two separate wills, one good the other evil--before Pelagianism was born, Augustine had already refuted it too. The will is not the simple, complete faculty that Pelagius had thought, but it is made up of several if not many conflicting desires. It is only under the influence of Grace that a human will receives the interior strength and resolve to will and to "do the good I ought." Simply stated, Pelagius had overestimated human nature and its innate ability to desire, think, do or be good, without God's constant help.
Speaking of St. Augustine, the Holy Father has an interesting take on the Saint and today's youth. It can be found on the Zenit website.