Nigel,
I'm not an attorney, least of all a French attorney, but the clearest explanation I've seen is contained in French regulations, specifically Article R413-17 of le code de la route (English translation below, the original French may be accessed by the link above):
II.-One's speed must be reduced:
1 ° When crossing or overtaking pedestrians, including those who have
exited their vehicle or bicyclists riding alone or in groups;
1 ° (amended) When crossing or overtaking any vehicle, immobilized or
traveling at low speed on a shoulder, an emergency lane or other pavement, equipped with the special lights mentioned in Articles R. 313-27 and R. 313-28 or whose driver makes use of his hazard warning
lights under the conditions provided for in the first paragraph of
Article R. 416-18;
2 ° When passing convoys at a standstill;
3 ° When crossing or passing public transport vehicles or
vehicles used for transporting children and subject to special
signage, while riders are either boarding or deboarding;
4 ° In all cases where the road does not appear to be completely
clear, or risks being slippery;
5 ° When visibility is insufficient (rainy weather and other precipitation, fog ...);
6 ° In curves;
7 ° In fast descents;
8 ° In sections of narrow or congested roads or roads lined with dwellings;
9 ° Approaching hill tops and intersections where visibility is not
assured;
10 ° When making use of special lights and in particular low beams;
11 ° When crossing the path of, or passing, animals.
No. 4 seems to suggest that in any case where the road is not completely free from precipitation (which could mean rain, ice, snow, or hail) or water or other substance from man's activities (say, water or foam on the roadway from firefighting or oil from a leak or spill), one must decrease speed or risk a fine and other measures, depending on circumstances.
No. 10 explains why high beams are mandatory in France outside towns (except when passing other drivers or where the road is lighted) but not allowed inside towns, because if you're not using high beams outside towns, you're supposed to be traveling at a reduced speed.
I hope that helps. I'd be curious if anyone else has a better reference. It's certainly vague.
I suspect, in practice, it's up to the police officer who has stopped you to attest to road conditions, so at some point it might be your word (and that of any passengers) against the officer's, if you choose to contest the fine.