Full Idea
The anti-haecceitist thesis (according to Adams's version) is that all thisnesses are reducible to, or supervenient upon, suchnesses.
Clarification
A 'suchness' is what is qualitative
Gist of Idea
Adams says anti-haecceitism reduces all thisness to suchness
Source
report of Robert Merrihew Adams (Primitive Thisness and Primitive Identity [1979]) by Robert C. Stalnaker - Mere Possibilities 3.5
Book Reference
Stalnaker,Robert C.: 'Mere Possibilities' [Princeton 2012], p.64