Robert Rauschenberg, like Autolycus in The Winter’s Tale, was a ‘snapper-up of unconsidered trifles’. Unlike Shakespeare’s character, however, he made them into art. Rauschenberg’s most celebrated piece, ‘Monogram’, on view in the grand retrospective of his work at Tate Modern, comprises, among other bits and pieces, a rubber shoe heel, a tennis ball, and a car tyre-plus-oil paint on stuffed angora goat.