 Poincaré’s views evolved over the years and in the debate with Russell. In the later period, he advanced a novel approach to predicativity, which, though informally sketched, is suggestive of later developments in definability and proof theory (see Feferman 1964, Heinzmann 1985). He no longer insisted on the vicious circularity of the definition involved in the contradictions; instead, he held the view that a predicative classification is characterized by invariance, i.e., that it cannot be affected by the introduction of new elements; by contrast, impredicative notions are subject to constant modification whenever new elements are introduced.