Reference. The Methodology of Positive Economics [friedman2007]

2007
Milton Friedman with contributions from Daniel M. Hausman
Philosophy of Economics: An Anthology
10.1017/CBO9780511819025.010
https://www.cambridge.org/core/books/philosophy-of-economics/methodology-of-positive-economics/CFF477DAB3196A749117CDC0F44E8160

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Note. Some varieties of idealisation [001F]

27 April 2026

I shall take idealisation to be the ‘intentional introduction of distortion into scientific theories’ (Weisberg, Idealization: 639) (presumably with non-malicious intent). The accuracy or realism of an idealisation is not, per se, necessarily of interest. It might mislead as to the preidctive inaccuracy entailed by the idealisation (Friedman, Methodology: § III.)

Example. Acceleration in a vacuüm and in the atmosphere

27 April 2026
J.P. Loo

In certain circumstances, we idealise the acceleration of a body on earth as if it were dropped in a vacuüm.

We could test the hypothesis by comparing the air pressure in a vacuüm and on earth. But this is ‘a foolish question by itself’. If we are dropping a ball from a building, its acceleration probably will closely approximate that in a vacuüm; but not a feather. So the putative accuracy of the antecedent idealisation is not, per se, a good guide to its legitimacy or usefulness. We should also consider the predictive inaccuracy it introduces.

Friemdan says that predictive accuracy is the ‘only relevant standard of comparison’. We need not go that far to appreciate the point that it is a relevant standard of comparison, and that the accuracy of the idealisation per se is not always relevant.

One lesson to draw from that example is that the purpose of an idealisation matters. Weisberg (Idealization: § 1) distinguishes ‘Galilean’ and ‘minimalist’ idealisation. The former distorts theories to simplify them, for computational tractability—‘to get traction on the problem’ on pragmatic grounds. The latter construts and studies theoretical models containing only the ‘core causal factors which give rise to a phenomenon’. In general, idealisations’ purposes are representational ideals, e.g. (Weisberg, Idealization: § 2)

inclusion rules [which] tell the theorist which kinds of properties of the phenomenon [are] of interest[; and] fidelity rules [concerning] the degrees of precision and accuracy with which each part of the model is to be judged.

For instance, completeness requires inclusion of all the properties of the target phenomenon, anything external giving rise to those properties, and an analogue of any structural or causal relationship within the model, all with arbitrary accuracy and precision. Simplicity can e.g. serve pedagogical purposes, or show the minimal conditions required to generate some property. Isolating ‘only…the factors that made a difference’ can help us to formulate or analyse more complex models. Predictive accuracy may be practically useful. And so on.

Related

Journal. Philosophy of Economics: An Anthology [peaa]