Textbooks must explain the economic process.
Description describes; science explains and provides for wise control. Scientific explanation gives the Why and How of phenomena in the form of relations of terms among themselves.
A distinction has been drawn between description and explanation. Description deals with things as related to us. Explanation deals with the same things as related among themselves. The two are not totally independent, for they deal with the same things and, as we have seen, description supplies, as it were, the tweezers by which we hold things while explanations are being discovered or verified, applied or revised. But despite their intimate connection, it remains that description and explanation envisage things in fundamentally different manners. The relations of things among themselves are, in general, a different field from the relations of things to us. … Not only are description and explanation distinct, but there are two main varieties of description. There are the ordinary descriptions that can be cast in ordinary language. There are also the scientific descriptions for which ordinary language quickly proves inadequate and so is forced to yield its place to a special, technical terminology. (But) both ordinary and scientific description are concerned with things as related to us, but both are not concerned with the same relations to us. The scientist selects the relations of things to us that lead more directly to knowledge of the relations of things among themselves. Ordinary description is free from this ulterior preoccupation. [CWL 3, 291-92/316-17]
An ‘accountant’s unity’… is a category used in (conventional) accounting. For Lonergan, (conventional) accounting generally denotes an enterprise within common sense which uses descriptive, as contrasted with explanatory terms (on these terms see CWL 3, 37-38/61-62, 178-79/201-3, 247-48/272-73). Insofar as that is true, the accountant’s unity is not an adequate index for the normative, explanatory analysis of the productive process. [CWL 15, 26, ftnt 26]
In brief Lonergan is looking for an explanation in which the terms are defined by the relations in which they stand, that is, by a process of implicit definition. … No doubt Keynes was an economist first and a methodologist second … Lonergan, for his part, is perhaps a methodologist first and an economist second, but he was able to push his economic reflections further than Keynes because he had a firmer grasp of the essentials of an effective theory. … Lonergan’s critique (shows that) … the emphasis shifts … to searching heuristically for the maximum extent of (functional) interconnections and interdependence; and that the variables (of the mechanism) discovered in this way might not resemble very much the objects (or the aggregates) (such as coincidental prices) which, in the first instance, (the non-methodologist) was thinking about. [Gibbons 1987]
Also see under main topic Key Notions the subtopic Postulating vs Explaining in macroeconomics