Category without objects

Some years ago (around 30?) I read a book where it was mentioned that one could define categories without (explicit) objects in the sense that objects are mimicked by identity morphisms. Unfortunately, I can not reconstruct what book it was. I know how this works. I would, however, like to have a reference. Best Uwe Wolter [For admin and other information see: http://www.mta.ca/~cat-dist/ ]