Shadow chancellor John McDonnell has given all his cabinet a copy of Cicero’s advice on how to win arguments. This is a very foolish move.

‘Rhetoric’ (same root as ‘orator’), or persuasive speaking, was the name of this activity. In the 4th century bc, Aristotle produced the definitive guide in his Art of Rhetoric, from which most of Cicero’s advice is drawn. His top tips included: work from the general (is this good in principle?) to the specific (is this example of it practical?). Examine any course of action under four headings: is it possible? Necessary? Advantageous? Honourable (i.e. just, moral, etc.)? Set up arguments from evidence, logic, likelihood, maxims (‘too many cooks’) and parallel examples (usually from the past). Most of all, present yourself as an honest person (êthos) and understand your audience so you know how to play on their emotions (pathos).

For Plato this was the work of the devil. The reason was that the actual training in rhetoric taught a man how to argue both sides of any case with equal ease, without considering where right and wrong lay. It was, in other words, a means to an end, whether the end was noble or ignoble. That will suit a man like McDonnell perfectly, but what should worry him is that rhetoric encourages listeners to question what they hear.

That seems to be the reason why discussions about rhetoric began to emerge in 5th century bc Athens, home of the world’s first and last democracy. The assembly of male citizens made all the decisions after listening to speeches from those willing to speak. The latter wished to persuade by any means, the former to know if the wool was being pulled over their eyes, and if so, how to remove it. The emergence of such discussions enlightened both sides.

Guides to winning arguments are by their very nature guides to defeating them. They may empower speakers, but they also empower the listeners. This could be fatal to the McDonnell-Corbyn project, a blinding golden ‘fleece’ if ever there was one.