Polly Toynbee tells us that Boris Johnson is being absurd in quoting from some obscure medieval source to justify tax cuts based upon the Laffer Curve:

The Laffer curve is a regular in his columns, though his latest pretentiousness name-checks another tax-cutting guru – an obscure 14th-century Islamic scholar, Ibn Khaldun.

A mere modicum of knowledge on the subject would explain why this particular reference is made. Here is Art Laffer himself talking about the Curve:

The Laffer Curve, by the way, was not invented by me. For example, Ibn Khaldun, a 14th century Muslim philosopher, wrote in his work The Muqaddimah: "It should be known that at the beginning of the dynasty, taxation yields a large revenue from small assessments. At the end of the dynasty, taxation yields a small revenue from large assessments."

Quoting Ibn Khaldun isn’t pretentiousness it’s recording both the intellectual history of the concept plus its obviousness. From, you know, the actual source of our modern day discussion of the point. Laffer also quotes Keynes:

When, on the contrary, I show, a little elaborately, as in the ensuing chapter, that to create wealth will increase the national income and that a large proportion of any increase in the national income will accrue to an Exchequer, amongst whose largest outgoings is the payment of incomes to those who are unemployed and whose receipts are a proportion of the incomes of those who are occupied...

Nor should the argument seem strange that taxation may be so high as to defeat its object, and that, given sufficient time to gather the fruits, a reduction of taxation will run a better chance than an increase of balancing the budget. For to take the opposite view today is to resemble a manufacturer who, running at a loss, decides to raise his price, and when his declining sales increase the loss, wrapping himself in the rectitude of plain arithmetic, decides that prudence requires him to raise the price still more--and who, when at last his account is balanced with nought on both sides, is still found righteously declaring that it would have been the act of a gambler to reduce the price when you were already making a loss.

These are well known points, sufficiently well known that Boris Johnson - not known as a great economist nor, often enough, someone greatly interested in economics - knows of them and even quotes at least one of them. Unknown to Polly Toynbee of course which is why we should perhaps regard askance her knowledge of matters taxation and economic.