Full Idea
'Valence' and 'gene' were perfectly clear long before anyone succeeded in reducing them, but it was their reducibility and not their clarity before reduction that showed them to be compatible with physicalism.
Gist of Idea
'Valence' and 'gene' had to be reduced to show their compatibility with physicalism
Source
Hartry Field (Tarski's Theory of Truth [1972], §5)
Book Reference
'The Nature of Truth', ed/tr. Lynch, Michael P. [MIT 2001], p.391