9 As I said then, the knowledge of each of these things resides within the conscience of all men, and we require no teacher to instruct us in these things; but the regulation of our conduct is left to our choice, and earnestness, and efforts. And why was this? But because if He had made everything to be of nature, we should have departed uncrowned and destitute of reward; and even as the brutes, who receive no reward nor praise for those advantages which they have naturally, so neither should we enjoy any of these things; for natural advantages are not the praise and commendation of those who have them, but of the Giver.
For this reason, then, He did not commit all to nature; and again, He did not suffer our will to undertake the whole burden of knowledge, and of right regulation; lest it should despair at the labour of virtue. But conscience suggests to it what ought to be done; and it contributes its own exertions for the accomplishment. That it is a good thing to be temperate, we all understand without difficulty; for the knowledge is of nature: but we should not be able without difficulty, without bridling lust, and employing much exertion, to practise the rule of temperance; for this does not come to us by nature as the knowledge does, but requires also a willing mind and earnestness.
And not only in this respect has He made the burden lighter for us, but also in another way again, by letting even some good dispositions exist naturally within us. For we are all naturally disposed to feel indignation along with those who are contemptuously treated, (whence it arises that we become the enemies of those who are insolent, though we ourselves may have suffered no part of the grievance,) and to sympathize in the pleasure of those who enjoy assistance and protection; and we are overcome by the calamities of others, as well as by mutual tenderness. For although calamitous events may seem to induce a certain pusillanimity, we entertain nevertheless a common fondness for each other.
And to this effect a certain wise man speaks significantly; “Every animal loves his like, and man his neighbour.”
Source: Homilies on the Statues (New Advent)