Some people believe that anyone who disagrees with them is either evil or stupid. Such people deny that it is possible for there to be reasonable disagreement. Many people would reject that but believe than anyone who disagrees radically with them is either evil or stupid. Such people deny that it is possible for there to be reasonable radical disagreement. This last position appears to be held by significant portions of Trump, Clinton, Brexit and Remain supporters. All of these people are wrong. Here is why:
1. If reasonable radical disagreement can be ruled out anywhere then it can be ruled out for the case of epistemic peers: people who are roughly equally rational, expert and knowledgable about a point at issue.
2. It is easy to find two philosophers who are epistemic peers where one will affirm and the other deny the existence of free will.
3. Such disagreements are about as radical as any disagreement could possibly be.
4. Both philosophers have examined everything they know to be relevant to the question at issue, including any belief or argument of their peer that can be communicated, and each has thought at depth and at length about the issue.
5. Since both are equally rational they will give equal weight to all the evidence that they share (which evidence includes their detailed knowledge of their disagreement and that it is disagreement with an epistemic peer).
6. So their disagreement cannot be adequately explained on the basis of communicable beliefs (even allowing for the general underdetermination of answers to philosophical questions by the considerations available).
7. Therefore we must allow of there being incommunicable beliefs (which, when true, are incommunicable insights).
8. So among the evidential base of their philosophical belief are states that are either incommunicable insights or states we are unable to distinguish from such insights but whose content is erroneous.
9. What can be communicated between them has not led either of them to realise that something they took to be an insight is an error.
10. Each philosopher’s opinion is not irrational given his evidence.
11. Therefore their radical disagreement is reasonable.
12. So radical disagreement cannot be ruled out anywhere. (1, 2, 13)
13. A forteriori, it cannot be ruled out for pairs of people who are not epistemic peers.
14. Most pairs of people who radically disagree are not epistemic peers.
15. Provided neither is generally irrational, and provided the opinion of each is not itself irrational given the evidence of each, any such pair of people can reasonably radically disagree.
Note that premiss 6 is a very strong assumption about rationality and yet despite its strength we can derive reasonable radical disagreement. If this premiss is given up and rationality does not determine equal weighing of evidence it is even easier since we could move directly to line 10.
This argument is conclusive so now if you disagree with me that people can radically disagree whilst being neither evil nor stupid then you are either evil or stupid.