I've been thinking about why good people disagree with each other about important things, like whether to support or oppose a particular law.  One, there may be some sort of mental deficiency or some crucial lack of knowledge. (Not truly common causes, in my opinion.) Two, there may be terminology issues. These may be legitimate differences in defining terms being used but sometimes I wonder if this is really more the side-effect of a basic disagreement rather than the cause. Three, and most important I think, there may be underlying differences in values (beliefs, morals, motivations, and even personality) that guide 'good' people to take opposite sides on important social debates. My questions for the philosophers here: A: is this a reasonable scheme to categorize disagreements among people? B: What have philosophers had to say about this topic?

Philosophers, especially lately, have had a lot to say about this topic. You might start with this SEP entry, and I recommend looking at this recent anthology. Fortunately (or unfortunately, if you wanted a short answer!), there's a large and growing literature on the topic. Enjoy.

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