Can I beg your indulgence? Here is a little parable (712 words). It is meant to undermine Hempel's requirement of maximal specificity in IS-explanation. I would be very interested to know how people react to it:
"Imagine an economist interested in the price elasticity of demand. She finds that price changes have different effects on demand for different kinds of commodity. For necessities such as staple food items, it has very little effect; for small conveniences such as dry-cleaning, the effect of price increases is linear; for luxury items with pretensions of exclusivity, it is inverted – the higher the price, the greater the demand – and so on. She also discovers that hitherto overlooked transaction-factors are relevant: elasticity takes on different values in business-to-business transactions, in on-line purchases, and so forth. Such additional determinants would interest our economist – though she may not have been aware of or interested in different categories of transaction until they turned up in her surveys, such factors are relevant to her question. She is interested in a complete economic theory of price elasticity, not just in some pre-selected set of factors. Her discipline defines the relevant features by its methodology; she does not define them by idiosyncratic interests.
"Let’s suppose that at a certain point in her investigation, the economist has discovered all the economic determinants – even those that she did not anticipate at first. But she has also stumbled upon a non-economic factor involved in some sales transactions. Say, for instance, that for reasons having to do with colour perception, some women’s willingness to tolerate a higher cost varies with the colour and size of the font in which the price of an item is displayed and with the kind of light shining on the price ticket. (Many women are more sensitive to colour differences than men.) In different stores, where the price happens to be displayed differently, price increases affect demand differently on average, because women are differently affected.
"I conjecture that the economist will not count this as an admissible factor. The effect arises situationally; it may vary simply with the prevailing light at different times of day, and with the purely accidental use of different price-tickets in different stores. Such accidental variation has nothing to do with the kinds of interaction that the economist is trying to understand. It affects elasticity, but to include its effects would distort the economist’s understanding of the transactional influences she is investigating. Because this data is irrelevant from her point of view; the economist would average over the perceptual variations – or perhaps even “correct for” them as government economic statisticians correct for seasonal variation in unemployment rates – rather than building them into her model. Of course, she may change her mind about this: she, or economists in general, may come to think that perceptual oddities are actually of economic interest (just as they have recently come to believe that the psychological oddities of consumers are of economic interest). However this may be, the point that I wish to make here is that there are usually some factors of this sort in the “special sciences” – factors that make a causal difference to the explanandum, but are deemed outside of the concern of the investigator.
"A moment ago, I mentioned “averaging over” perceptual variation. This is what I mean. The economist is trying to predict demand: this is the outcome that interests her. Her model predicts how demand varies with price, given certain other characteristics of transactions. Let C be a type or class of transactions that are indistinguishable from an economic point of view. Every transaction in C is, in other words, the same as every other with respect to the factors that interest our economist. The demand for a commodity will be the sum of the demand for it in various types of transaction. However, within a given type, demand will depend (in part) on how transactions happen to be influenced by the perceptual factors just mentioned. The economist ignores this variation in C: she simply lumps these transactions together, and calculates elasticity for C as an average over different modes of display, men and women, and different lighting conditions. In this manner, the economist tolerates a source of variation in instances of C. Consequently, elasticity is predicted only probabilistically. Given an increase of price, there is a probability function associated with various values of demand for C. Some of that uncertainty comes from a suppression of a causally relevant, but theoretically inadmissible, factor."