Peter Singer
AGE: 60
OCCUPATION: Ethicist at Princeton University
NUMBER OF TIME COVERS: 0
PREVIOUS APPEARANCES ON THE TIME 100: 1
PRO: His 1975 book Animal Liberation practically founded the eponymous movement. Whether you agree or disagree with him, his critique of the treatment of animals in the post-industrial age -- from vivisection to butchery -- is rich food for thought.
CON: Vegetarianism, which Singer advocates as the only ethical response to factory farming, still remains a marginalized movement, with relatively few followers compared to the huge number of carnivores.