The great Italian humanist Lorenzo Valla (1406-1457) took great pride in his constant need to attack his forebearers, as well as his contemporaries. Few escaped the wrathful attentions of his pen. Yet Valla saw himself as an upholder of the classical virtues, and for him, criticism was a form of moral duty. He said in 1440 in one letter to Joan Serra:
And if there are others I have criticized, I should want to give a similar answer: I feel the confidence to correct them when I see that those of an earlier generation who are quite rightly hailed as authorities do not share their views and, so far as it is within their power, silently contradict them…Who has ever written on any field of learning or science without criticizing his predecessors? What reason could there be for otherwise writing, if not to castigate the errors, omissions, or excesses of others? 
But Valla was a genius, and he could back up his words. He poured scorn on the idea that our predecessors should be held in uncritical esteem, or that the old should automatically receive the respect of the young. To this end, he quoted Horace, who said:
Or because they’re ashamed to listen to younger men, and admit
In old age that what they learned as beardless youths should be thrown away.
Few, if any of the humanists possessed his linguistic and scholarly abilities, and he knew it. His achievements did invite some resentment. Francesco Filelfo, a brilliant man in his own right, disapproved of Valla’s somewhat arrogant manner, and said “He [Valla] wages a relentless and wicked war against learned men, appointing himself the master of every art.”
Valla enjoyed to the fullest the license that genius provides, but most of us remain grounded to the earthly tethers of modest ability. We find it more useful to listen than to criticize; listening activates the critical faculties, which then must be held in check by discipline and prudence. When we see a man making egregious mistakes, our first thought should be, as Seneca says,
Have we ourselves never been guilty of such an act? Have we ever made the same mistake? Is it right for us to condemn this? 
Silence in the face of obvious foolishness or stupidity takes discipline. The proper question we should ask ourselves in such situations is this: are these mistakes instructive to me? Can I learn anything from observing the follies or misfortunes of this other person? For there is no doubt that the one of the greatest regrets of the old is this: not to have become wiser before having become old. To become wise while still in a position to make use of youth: this is without doubt a great achievement. The best way to make progress in this goal is to add the mistakes of others to the catalog of our own mistakes, so that the book of errors becomes that much thicker.
Listening itself is an art. Everyone praises those who know how to speak, but few praise those who know how to listen. In listening, we must focus on the larger themes, and discard the petty details. We must assume the correct posture, being aware that the receipt of aural information is conditional on proper physical deportment. A speaker should treat our minds as a beehive, says Plutarch, “who cleanses it with the smoke of caustic speech.” Speech that provides no opportunity for stimulus or response is close to useless.
It is our responsibility to prepare our receptors adequately for the task of reception. No slouch, sliding about in his chair, ever made a good listener. The philosopher Pythagoras spent a great deal of time with his novices on the proper means of listening to instruction. He also forbade his novitiates from speaking in the early stages of instruction. He found this rule to be necessary from long experience. Newcomers to philosophy treat ideas like puppies do a new bone: they chew on them intensely, but are inclined to be scatter-brained, and fail to control them properly.
The origins of excessive criticism are these: arrogance, anger, and shame. That man who oozes hollow conceit and ostentatious self-aggrandizement finds it amusing to toy with others, a cat might toss about a household toy. Anger is a tempestuous emotion, and generates criticism easily as it reflects itself outward. The best corrective for criticism arising from anger is delay; that is, we should try not to make responses to matters when angry, but should delay those responses to permit cooler reflection.
Shame is the final, and most profound, source of unjustified criticism of others. This has its origin in psychological causes: we are accustomed to lash out and attack that which reminds us of our innermost shameful selves. We find it expedient to cover up our debts by castigating our intellectual creditors. The faults and vices of others triggers in us a reminder of our former selves, the self which we seek to banish or repress. We must drive away this unpleasant reminder of our own faults, and this hostility finds expression in bitter criticism.
Criticism may in fact be likened to a waste product of our bodies, in this sense: it may be tolerable to ourselves, but the sight or sound of it to others will be unpleasant. Countless great men have marred the pages of their works with tasteless criticism of others that, to them at least, may have looked at one time clever or amusing, but which now, with the passage of time, only appears pathetic.
And yet even unjustified criticism, although damaging to its user, can serve a useful purpose to the receiver. Such abuse should be seen as part of life’s bitter rituals, to be endured until we are on our death-beds. It is also good training to learn to stand up for oneself in face of unwarranted abuse, and to return word for word in any verbal contest. Such confrontations sharpen the faculties, and serve as cleansing experiences. “Neither a bath,” said the Stoic philosopher Ariston, “nor a discussion are any good unless they are cleansing.”
Originality invites criticism. Yet criticism should not automatically be seen as proof of originality. Everyone believes himself and his ideas to be original, yet in practice very few actually are. This is because, as we view the nearly limitless expanse of history, human nature has shown a remarkable degree of consistency. Little of what we encounter today has not already been encountered numberless times before. It is difficult to be original without also being wrong; and there is little new in the world of ideas outside of arrangement and ornament.
Finally, it is best to avoid criticism for the corrosive effect it can have on our psyches. Wild and intemperate speech, punctuated with castigation, carries the taint of the circus showman. Amusing in its puffery, it quickly loses all power to instruct or entertain, and reduces its originator to unheroic dimensions. More in this life has been gained by patient promotion and silent absorption than has been achieved by nitpicking criticism. It is the doer of great deeds whom history celebrates, and not the critic.
Erasmus, in a 1519 letter to Martin Luther, had these potent words to say about moderation in speech and the pernicious effects of invective:
It might be wiser of you to denounce those who misuse the Pope’s authority than to attack the Pope himself. So it is also with sovereigns and princes. Old institutions cannot be cast aside in an instant. Reasoned argument may do more good than wholesale condemnation. Avoid all appearance of sedition. Keep your head. Do not become enraged. Do not hate anyone. Do not be excited with the noise you have made…
And so we can see that the criticism of the reformer, at its productive best, operates as the engine of change; but this engine must be moderated and controlled by tradition. The machine, left to its own devices and without adequate supervision, inevitably destroys itself.
So criticism in this way can be seen to have both positive and negative effects. For us, when faced with few options, we can only endure bad things until better ones come along. As Celsus says, (Prooemium De Medicina 18): Occursurum enim vitio dicunt eum, qui originem non ignorat. And this means, “They say that he who is not ignorant of the origin of vice, will be most able to respond to its challenge.”
 Cook, B. (ed.) Lorenzo Valla: Correspondence, Cambridge: Harvard Univ. Press, 2013, p. xvi.
 Horace, Epist., 2.1.84-85: Vel quia turpe putant parere minoribus, et quae imberbes didicere senes perdenda fateri.
 Seneca, De Ira, II.28.8: Numquid et ipsi aliquid tale commisimus? Numquid sic erravimus? Expeditne nobis ista damnare?
 Cf. Durant, W. The Reformation, New York: Simon & Schuster, 1957, p. 430.
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