 André Ariew (1996; 1999; 2007) has developed a more specific version of the environmental invariance account of innateness. Following Stich (1975) and Sober (1988), Ariew maintains that the primary job for the concept of innateness – not just in psychology, but in the life sciences generally – is to explain environmental invariance. The problem with the norm of reaction account, Ariew objects, is that it merely redescribes this pattern rather than explaining the processes that generate it. In an effort to explain invariance, Ariew invokes C. H. Waddington’s idea of environmental canalization. An environmentally canalized trait is buffered against certain types and amounts of environmental variation over stages of its development. Waddington described this phenomenon using the metaphor of an “epigenetic landscape” which consists of a ball rolling down a contoured surface (Figure 4). The surface is sloped like a pinball machine. The path traced by a ball as it rolls through a particular valley represents the trajectory of a developing trait. The amount of “shaking” required to make the ball jump out of one valley and into another depends on the overall shape of the landscape. The deeper (more canalized) the valley, the more buffered the trait is against environmental changes (or genetic changes, but Ariew focuses on environmental canalization rather than its complement, genetic canalization). His proposal is simply that a trait should be considered innate to the extent that it is environmentally canalized in Waddington’s sense. One advantage of this definition, he adds, is that it represents innateness as a matter of degree rather than as an all-or-nothing property.