It is just too comforting to think that those you disagree with are ignorant or steeped in moral turpitude, preferably both.
Milton Friedman argued that people agree on most social objectives, but they differ often on the predicted outcomes of different policies and institutions.
This leads us to Robert and Zeckhauser’s taxonomy of disagreement:
Positive disagreements can be over questions of:
1. Scope: what elements of the world one is trying to understand?
2. Model: what mechanisms explain the behaviour of the world?
3. Estimate: what estimates of the model’s parameters are thought to obtain in particular contexts?
Values disagreements can be over questions of:
1. Standing: who counts?
2. Criteria: what counts?
3. Weights: how much different individuals and criteria count?
Any positive analysis tends to include elements of scope, model, and estimation, though often these elements intertwine; they frequently feature in debates in an implicit or undifferentiated manner.
Likewise, normative analysis will also include elements of standing, criteria, and weights, whether or not these distinctions are recognised.
The origin of political disagreement is a broad church indeed in a liberal democracy. Those you disagree with are not evil, they just disagree with you. As Karl Popper observed:
There are many difficulties impeding the rapid spread of reasonableness. One of the main difficulties is that it always takes two to make a discussion reasonable. Each of the parties must be ready to learn from the other.
A nice case against the notion of the ignorance and moral turpitude of your opponents is the obituary by Brad DeLong for Milton Friedman which was as good as any written saying:
His wits were smart, his perceptions acute, his arguments strong, his reasoning powers clear, coherent, and terrifyingly quick. You tangled with him at your peril. And you left not necessarily convinced, but well aware of the weak points in your own argument
Milton Friedman’s thought is, I believe, best seen as the fusion of two strongly American currents: libertarianism and pragmatism. Friedman was a pragmatic libertarian. He believed that–as an empirical matter–giving individuals freedom and letting them coordinate their actions by buying and selling on markets would produce the best results… For right-of-center American libertarian economists, Milton Friedman was a powerful leader. For left-of-center American liberal economists, Milton Friedman was an enlightened adversary. We are all the stronger for his work. We will miss him.