For indeed it is possible that a man may think well, and yet not be able to express his thoughts elegantly; but for any one to publish thoughts which he can neither arrange skilfully nor illustrate so as to entertain his reader, is an unpardonable abuse of letters and retirement: they, therefore, read their books to one another, and no one ever takes them up but those who wish to have the same licence for careless writing allowed to themselves.

[Fieri autem potest, ut recte quis sentiat et id quod sentit polite eloqui non possit; sed mandare quemquam litteris cogitationes suas, qui eas nec disponere nec inlustrare possit nec delectatione aliqua allicere lectorem, hominis est intemperanter abutentis et otio et litteris. Itaque suos libros ipsi legunt cum suis, nec quisquam attingit praeter eos, qui eandem licentiam scribendi sibi permitti volunt.]

Marcus Tullius Cicero (106-43 BC) Roman orator, statesman, philosopher

Tusculan Disputations [Tusculanae Disputationes], Book 1, ch. 3 (1.3) / sec. 6 (45 BC) [tr. Yonge (1853)]
    (Source)

Source (Latin). Alternate translations:

Now it is possible, that one may have true Conceptions, and yet not be able to express his Notions in proper Terms; but for a man to commit his thoughts to writing for the publick, who can neither put them in due method, nor illustrate them with clear Proofs, nor by any delightful Ornaments entertain his Reader, is the part of one that at no rate abuses his own time, and the benefit of Writing. Here∣upon they read their own Books among themselves, nor doth any one else meddle with them, but they that expect allowance to write after the same loose fashion.
[tr. Wase (1643)]

For indeed it may be that a man may think well, and yet not be able to express his thoughts elegant; but for any one ot publish thoughts which eh can neither methodize, nor illustrate nor entertain his reader, is an unpardonable abuse of letters and retirement: they, therefore, read their books to one another, which were never taken up by any but those who claimed the same privilege of writing.
[tr. Main (1824)]

For it may very well happen, that a man may think rightly, and yet be unable to give utterance to his sentiments with sufficient elegance. But, for any one to consign his thoughts to letters, who can neither arrange them with method, nor make them intelligible by illustration, nor attract the reader with any delight, is the part of a man who rashly abuses both his leisure and literature. And, therefore, let them read their books themselves with their friends; nor let them be touched by any, except by those who are like to need the same indulgence for the same license in writing.
[tr. Otis (1839)]

One may think correctly, yet be unable to give elegant expression to what he thinks; and in that case for a man to commit his thoughts to writing when he can neither arrange them, nor illustrate them, nor attract readers by anything that can give them delight, is the part of a man who outrageously abuses both leisure and letters. Such writers read their own books with their intimate friends, nor does any one else touch them except those who crave for themselves like liberty of writing.
[tr. Peabody (1886)]

Even when they have their arguments in order, they don't express them with any flair. They waste their free time -- and do a discredit to literature -- when they commit thoughts to writing without knowing how to arrange or enliven them or give any pleasure to the reader. And so they just end up reading each other's books! No one pays attention to them except people who hope to qualify for the same writer's licence.
[tr. Habinek (1996)]

But it can happen that someone may have a good thought which he cannot express well.
[tr. @sentantiq (2016)]

It is possible for a man to hold the right views but be incapable of expressing these with any elegance; but that anyone should entrust his thoughts to writing, without the ability to arrange them or to express them with clarity, or to attract the reader by offering him some pleasure, is characteristic of a man who is making an ill-disciplined misuse of both leisure and writing. The result is these fellows read their own books to their own circle and no one touches them except those who wish to be permitted the same freedom in writing.
[tr. Davie (2017)]