What does it mean for philosophical writing to be great, significant literary works?

Question

British philosopher Nigel Warburton says the same thing in his books. Emboldenings are mine. I know that some French literature syllabi include Camus and Sartre. But at least in English, I have never seen philosophy books in any school or university literature syllabus!

What exactly does Warburton mean by great, significant literary work? How can philosophy texts be literature too?

The history of political philosophy includes many of the greatest and most widely studied works of philosophy, from Plato's Republic through to John Rawls's A Theory of Justice. In this book we are focusing on seven works by indisputably great thinkers. These books are intrinsically interesting, in some cases qualify as significant works of literature, and, most importantly, contain ideas that have a continuing relevance beyond their original contexts of composition. Our book has been written in the firm belief that studying the history of philosophy should not be like a visit to a dusty museum of superseded thought, but rather a challenging and invigorating engagement with the ideas of the great thinkers of the past.

Reading Political Philosophy Machiavelli to Mill, page vii.

These are known as the Platonic Dialogues and are great works of literature as well as of philosophy – in some ways Plato was the Shakespeare of his day.

A Little History of Philosophy, 2011, page 4.

The history of philosophy is a fascinating and important subject in its own right, and many of the classic philosophical texts are also great works of literature: Plato’s Socratic dialogues, René Descartes’s Meditations, David Hume’s Enquiry Concerning Human Understanding, and Friedrich Nietzsche’s Thus Spake Zarathustra, to take just a few examples, all stand out as compelling pieces of writing by any standards.

Philosophy: The Basics, 2012 5th Edition, page 2.

This book consists of thirty-two chapters, each focused on a single great philosophical book. The point is to introduce each book, bringing out its most important themes. The books dealt with here are worth reading today because they engage with philosophical problems that are still worth discussing, and because they continue to offer insights. Apart from that, many of them hold their own as great works of literature.

Philosophy: The Classics, 2014 4th edition, page 1.

Answer 1

Strictly speaking, the answer is already in some of the quotes from Warburton (or his co-editors, in some cases).

For example, in A Little History of Philosophy (emphasis mine):

These are known as the Platonic Dialogues and are great works of literature as well as of philosophy – in some ways Plato was the Shakespeare of his day.

I can think of two ways of interpreting this. First, Plato wrote dialogue that is captivating in the way the dialogue in Shakespeare's plays is captivating. (I'm not saying "as captivating as Shakespeare's dialogues" because I'm not certain that this is what Warburton meant.) Second, Shakespeare's stylised language is one of the main reasons why Shakespeare is still considered a great author, and Plato's dialogues are also worth reading for their language.

In Philosophy: The Basics (emphasis mine):

Plato’s Socratic dialogues, René Descartes’s Meditations, David Hume’s Enquiry Concerning Human Understanding, and Friedrich Nietzsche’s Thus Spake Zarathustra (…) all stand out as compelling pieces of writing by any standards.

This quote points into the direction of the second interpretation above. Since these works, unlike works of fiction, aren't compelling through such features as a plot or fascinating characters, it is most likely their language that makes them worth reading as literature.

(When I was at secondary school, pupils learning Greek read Plato's dialogues in class, just like they read, e.g. Polybius. And when I was studying German at university, one of the works we read was Nietzsche's Also sprach Zarathustra. So the idea of reading works of philosophy from a literary point of view is not far-fetched at all.)