Contributed by Leslie Roberts / The exhibition “Abstract by Definition” at Art Cake celebrates the 90th anniversary of the American Abstract Artists (AAA). The show is subtitled “An Index,” but is not one in the usual sense – not, that is, an itemized set of categories, styles, intentions, or formal languages defining abstract art. Curator Saul Ostrow has instead organized groups of several works – usually four. This installation effectively highlights the particular qualities of each piece, and emphasizes the diversity of what we call contemporary abstraction.