Tom Regan, an American animal right philosopher. “10 Reasons AGAINST Animal Rights and Their Replies”. Retrieved May 6th, 2008 – “4. Where do you draw the line? If primates and rodents have rights, then so do slugs and amoebas, which is absurd.
Reply: It often is not easy to know exactly where to “draw the line.” For example, we cannot say exactly how old someone must be to be old, or how tall someone must be to be tall. However, we can say, with certainty, that someone who is eighty-eight is old, and that another person who is 7’1″ is tall. Similarly, we cannot say exactly where to draw the line when it comes to those animals who have a psychology. But we can say with absolute certainty that, wherever one draws the line on scientific grounds, primates and rodents are on one side of it (the psychological side), whereas slugs and amoebas are on the other — which does not mean that we may destroy them unthinkingly.”