Full Idea
In drawing 'countefactual' conclusions we can be thought imaginatively to vary the conditions under which the substance, set-up etc. is manipulated or stimulated, while maintaining constant our conception of the nature of the being in question.
Gist of Idea
In counterfactuals we keep substances constant, and imagine new situations for them
Source
Rom Harré (Laws of Nature [1993], 2)
Book Reference
Harré,Rom: 'Laws of Nature' [Duckworth 1993], p.46
A Reaction
Presumably you could vary the substance and keep the situation fixed, but then the counterfactual seems to be 'about' something different. Either that or the 'situation' is a actually a set of substances to be tested.