9 The first point to be considered is whether you ought to live by yourself or in a monastery with others. For my part I should like you to have the society of holy men so as not to be thrown altogether on your resources. For if you set out upon a road that is new to you without a guide, you are sure to turn aside immediately either to the right or to the left, to lay yourself open to the assaults of error, to go too far or else not far enough, to weary yourself with running too fast or to loiter by the way and to fall asleep.
In loneliness pride quickly creeps upon a man: if he has fasted for a little while and has seen no one, he fancies himself a person of some note; forgetting who he is, whence he comes, and whither he goes, he lets his thoughts riot within and outwardly indulges in rash speech. Contrary to the apostle's wish he judges another man's servants, puts forth his hand to grasp whatever his appetite desires, sleeps as long he pleases, fears nobody, does what he likes, fancies everyone inferior to himself, spends more of his time in cities than in his cell, and, while with the brothers he affects to be retiring, rubs shoulders with the crowd in the streets.
What then, you will say? Do I condemn a solitary life? By no means: in fact I have often commended it. But I wish to see the monastic schools turn out soldiers who have no fear of the rough training of the desert, who have exhibited the spectacle of a holy life for a considerable time, who have made themselves last that they might be first, who have not been overcome by hunger or satiety, whose joy is in poverty, who teach virtue by their garb and mien, and who are too conscientious to invent— as some silly men do— monstrous stories of struggles with demons, designed to magnify their heroes in the eyes of the crowd and before all to extort money from it.
Source: Letters (New Advent)