p31.
Otto Weininger, “Play and Early Childhood”, in Paul F. Wilkinson, ed. (1980), In Celebration of Play, pp 43–62 (quotation from pp 51, 53 and 61, abridged). For a review of object play among young primates, see Jacklyn K. Ramsey and William C. McGrew, “Object Play in Great Apes”, in Anthony D. Pellegrini and Peter K. Smith, eds. (2005), The Nature of Play, pp 89–135.