But Thamus replied, "Most ingenious Theuth, one man has the ability to beget arts, but the ability to judge of their usefulness or harmfulness to their users belongs to another; and now you, who are the father of letters, have been led by your affection to ascribe to them a power the opposite of that which they really possess.Thamus is warning of three things: 1) New inventions will be sued in ways different than anticipated by the inventor, 2) that writing will not serve to enhance memory, and 3) Easy access to information will foster an inclination towards the appearance of knowledge and wisdom and undermine the actual pursuit of knowledge and wisdom.
For this invention will produce forgetfulness in the minds of those who learn to use it, because they will not practice their memory. Their trust in writing, produced by external characters which are no part of themselves, will discourage the use of their own memory within them. You have invented an elixir not of memory, but of reminding; and you offer your pupils the appearance of wisdom, not true wisdom, for they will read many things without instruction and will therefore seem to know many things, when they are for the most part ignorant and hard to get along with, since they are not wise, but only appear wise.
Postman has a similar read. He points out that
In fact, there is even one error in the judgment of Thalmus, from which we may also learn something of importance. The error is not in his claim that writing will damage memory and create false wisdom. It is demonstrable that writing has had such an effect. Thalmus' error is in his believing that writing will be a burden to society and nothing but a burden. For all his wisdom, he fails to imagine what writing's benefits might be, which, as we know, have been considerable. We may learn from this that it is a mistake to suppose that any technological innovation has a one-sded effect. Every technology is both a burden and a blessing, not either-or, but this-and-that.I agree. It is one of the most common errors in argument - either all the costs are shown and none of the benefits (if you are against the proposed change) or all the benefits are shown and none of the costs (if you are for the proposed change). Both forms are profoundly wrong and disrespectful of the audience but they are a routine form of argument. I suspect they are not always maliciously intended. Keen enthusiasts can blind themselves to the costs of a proposal and Casanovas equally blind themselves to benefits. They simply do not see both sides of an argument, and hence are not competent to argue.