Jonathan Portes describes his encounter with the Treasury Select Committee:

None of the questioners on this topic seemed at all interested in why I make the arguments I do, nor were they prepared to put forward any countervailing evidence of their own. They didn't define "credibility"; they didn't specify what the "incredible" counterfactual would look like; they didn't try to explain why "credibility" should matter from a theoretical perspective; they didn't try to present any empirical evidence that "credibility" had in fact resulted in lower gilt yields.

There's a reason for this. When the political class use the word "credibility", they don't intend it to be a testable proposition. It is, instead, a political "X factor" - something ill-defined but nevertheless desirable in leaders. So, for example, when the Telegraph asks "is Labour's EU policy credible?" it could have replaced "credible" with "reasonable" or "correct". But to have done so would have invited rational analysis in a way that "credible" does not.

Used this way, "credibility" serves an ideological function, in three ways:

1.It is used to define which policies are acceptable to the capitalist class, which fall within the Overton window. Austerity and "sound money" are "credible" even if they are not necessarily in the short-term interests of all capitalists. I suspect that this is the sense in which Jesse Norman was using the term. He and Jonathan were divided by a common language.

2. There's a dog whistle element here. Take the question: "is Ed Miliband a credible leader?" Objectively speaking, it's a silly question. His MSc in economics makes him more academically qualified than almost all previous Prime Ministers, and in 2015 he'll be older than Cameron or Blair were when they became PM. But objectivity isn't the point. By "credible" people mean "not too left wing" or geeky or (I fear) Jewish.

3. "Credibility" helps personalize politics and, in doing so, divert attention away from impersonal power structures. What Richard Sennett wrote almost 40 years ago is perhaps as true now as it was then:

A political leader running for office is spoken of as "credible" or "legitimate" in terms of what kind of man he is, rather than in terms of the actions or programmes he espouses. The obsession with persons at the expense of more impersonal social relations is like a filter which discolours our rational understanding of society; it obscures the continuing importance of class in advanced industrial society. (The Fall of Public Man, p4)