(This is a slightly improved version of a comment I made to a text by Duncan Richter on his blog, Language goes on holiday. The passage I'm commenting on is the following:
He [Stanley Cavell] questions whether anyone not crazed could make such a comparison. I think they could. It's true that there are obvious and crucial differences, and that making the comparison risks insulting the victims of Nazi genocide. But ignoring the similarities also risks being insensitive to what they suffered.)
I've also been puzzled by the claim that it would be crazy to compare factory farming to the Holocaust, or that you never should or even that you cannot make such a comparison. Of course you can. You can compare any two things you like. Whether it would be morally offensive, crazy or unintersesting to do so, is to a large extent a question of what you're trying to do with the comparison. Sure, we've seen some crude examples of this, from both racists and animal activists, but the problem isn't that they find similarities between factory farming and the Holocaust, but what kind of similarities they see and the point they're trying to make with them.
I think we can compare what's happening on modern farms with the concentration camps in order to make some points -- in order to drive the horror (at both places) home, say, without necessarily offending anyone. Many victims of the nazis have indeed done this themselves. They've lamented that the nazis treated them like slaughter animals. A description, I guess, most people don't find crazy at all, but spot on.
If you, on the other hand, are unaware that this is an emotional and moral mine field and simply imply that the two instances are identical in every (morally) relevant sense -- or perhaps you are aware and still make this claim -- you're sure to offend a lot of people. This would perhaps be crazy, and you shouldn't do it. It would be an insult the jews, an exaggeration (or a misframing) of the wrongness of what we're doing to animals, or both.