The American economist Paul Romer has recently written of “mathiness”, by analogy with “truthiness”, a term coined by American talk show host Stephen Colbert. Truthiness presents narratives which are not actually true, but consistent with the world view of the person who spins the story. It is exemplified in rightwing fabrications about European health systems — their death panels and forced euthanasia — and in some activists’ support for alleged rape victims even when their allegations are unsupported by evidence . Mathiness is a similar use of algebraic symbols and quantitative data to give an appearance of scientific content to ideological preconceptions.

It is characteristic of science to give precise meaning to concepts and the basis of their measurement. Every careful person equipped with a reliable thermometer will make the same reading of temperature. There are alternative scales, Fahrenheit and Celsius, but both record the same thing. We coin expressions like “it feels cold” to acknowledge that subjective experience of temperature may differ from objective fact.

Economics is genuinely harder. National income is a more complicated concept than temperature, and there are plausible alternative sets of rules for calculating it. Serious minded statisticians have spent many years discussing these issues, and there is now a UN-sponsored standardised system of national accounts. Given the same underlying observations, most well-trained officials will come up with very similar answers to the question “what is national income?”.

But it is easy to write a mathematical symbol without giving thought to what observable fact in the real world corresponds to that symbol, or whether there is such an observable fact at all. The measurement of capital has always been controversial, in a way the measurement of national income is not, in large part because Karl Marx chose that term for the title of his book.

Half a century ago, the measurement of capital was at the centre of an ideologically driven debate, the so-called “Cambridge capital theory controversy” between a group led by the Cambridge university Marxist economist Joan Robinson, and American scholars led by MIT professor Robert Solow. A debate which Solow won easily because of the care he took to specify both his models and the relevant data. But recently capital measurement and ideology have again become intertwined, through the attention given to the work of Thomas Piketty, with serious questions raised about the relationship between his data, his theory and the political stance which motivates his work.

In discussing “mathiness”, Mr Romer makes a distinction between what he calls “Feynman integrity” and “Stigler conviction”. For the physicist Richard Feynman, science involved “a kind of leaning over backwards. For example, if you’re doing an experiment, you should report everything that you think might make it invalid — not only what you think is right about it.” This is a proper aspiration, though an idealised view of science that few scientists actually practise. For George Stigler, a founder of the modern Chicago school of economics, “the successful inventor is a one-sided man. He is utterly persuaded of the . . . correctness of his ideas and he subordinates all other truths because they seem to him less important than the general acceptance of his truth.”

The distinction, made by Isaiah Berlin in his essay on Tolstoy’s view of history, is between “foxes” who know many little things and “hedgehogs” who know one big thing. The strange thing about economics is that because it spans both science and politics, both characteristics — careful analysis and effective polemic — are required. The geniuses of the subject, such as Adam Smith and JM Keynes, combined both. The practitioners of mathiness lack skill in either.

This article was first published in the Financial Times on October 7th, 2015.