Mortimer Adler, How to Read a Book:

You must be able to say, with reasonable certainty,

“I understand,”

before you can say any one of the following things:

“I agree,” or

“I disagree,” or

“I suspend judgment.”

For those who don’t do this, he says:

There is actually no point in answering critics of this sort.

The only polite thing to do is to ask them to state your position for you, the position they claim to be challenging.

If they cannot do it satisfactorily, if they cannot repeat what you have said in their own words, you know that they do not understand, and you are entirely justified in ignoring their criticisms.

Alder goes on:

When you find the rare person who shows that he understands what you are saying as well as you do, then you can delight in his agreement or be seriously disturbed by his dissent.