Peter Lupu left the following comment which deserves to be separately posted. I supplement Peter's thoughts with a quotation from Mary Midgley and some commentary.
In philosophical discourse the phrase "I do not understand" when stated about a philosophical position can mean either
(i) this position is so obscure that there is nothing in it to understand; or
(ii) this position is subject to several obvious objections (which I need not spell out) and therefore I fail to see how anyone can hold and/or propose it; or
(iii) this position is so difficult, abstract, and/or complex that I am unable to wrap my head around it.
Sense (iii) is not philosophical trash-talk. It is typically stated by a philosophical novice who really does not yet grasp the nature of philosophical positions or by a professional who is grappling with a genuinely difficult position and attempts to make sense of it.
Senses (i) and (ii), on the other hand, are too often used by opponents of a position as philosophical trash-talk. Their purpose is to intimidate the proponents of a position. The method goes something like this.
Example of Philosophical Trash-Talk:
"You and I agree that I am not a philosophical novice; given this assumption, if your position were not irreparably obscure, I would understand it; I do not understand it; therefore, it is irreparably obscure."
Now the proponent of the position so challenged has two options: he can defend the coherence of his position or else he must challenge the credentials of the opponent who uses a version of trash-talk exemplified above. Since many gentle souls would prefer not to opt for the later option, they are forced to defend the coherence of their position against challenges not yet stated. This achieves the intended purpose of the opponent to turn the burden on the proponent without having to do much except trash-talk.
Trash-talk has no place in philosophical discourse. A phrase such as "I do not understand" should be used only in sense (iii) either by a philosophical novice or by a professional who uses it to express their genuine effort to understand a difficult position and give it the most charitable reading. If a professional uses it in any other sense, they are trash-talking which, I hope we all agree, betrays the essence of philosophical inquiry.
Mary Midgley in The Owl of Minerva: A Memoir, Routledge, 2005, p. 13, reminisces about her headmistress, Miss Annie Bowden:
I also remember something striking that she had said when I had complained that I knew the answer to some question but I just couldn't say it clearly. 'If you can't say a thing clearly,' she replied, 'then you don't actually know what it is, do you?' This is a deep thought which I have often come back to, and it is in general a useful one. It lies at the heart of British empiricism. Though it is not by any means always true, I am glad to have had it put before me so early in life. It's a good thought to have when you are trying to clarify your own ideas, but a bad one when you are supposed to be understanding other people's. Philosophers are always complaining that other people's remarks are not clear when what they mean is that they are unwelcome. So they often cultivate the art of not understanding things -- something which British analytic philosophers are particularly good at. (Bolding added.)
My added emphasis signals my approbation.
We owe it to ourselves and our readers to be as clear as we can. But the whole point of philosophy is to extend clarity beyond the 'clarity' of everyday life and everyday thinking. The pursuit of this higher clarity, the attempt to work our way out of Plato's Cave, results in a kind of talking and thinking that must appear obscure to the Cave dweller. Well, so much the worse for him and his values. To demand Cave clarity of the philosopher is vulgar and philistine.