Belief is more psychologically distant than opinion because one’s own person is near, others’ distant, and belief incorporates (averages) others’ opinions, which are disregarded in rational opinion formation. Construal-level theory would indicate that we perceive others as acting for their beliefs and ourselves as acting for our opinions, that is, we see ourselves as engaged in deliberation when we see our counterparts engaged in action. The spiral ensues in which parties to deliberation misperceive the other as advancing an agenda rather than engaging in good-faith deliberation, forcing each to reciprocate because the party engaging in action controls through benefiting from the distraction.
Beliefs and opinions are different entities, not just different functions that entities serve. We typically ascribe beliefs to others to describe and predict their conduct. Beliefs are far-mode constructions that must be grounded in an inbuilt template—since belief ascription is humanly universal—an idealization, which reality only approximates. If you believe that “She’s telling the truth,” your actions will comport with the absence of suspected untruthfulness but only to a point. Belief is a matter of degree, based on how close the template and reality match.
Belief is a primitive intuition regarding others, but applying the concept of belief to oneself doesn’t come naturally. Ferreting out the contours of unarticulated belief is what gives insight-based psychotherapy its power. Although the thinking comes easily that another agent is deceiving himself about what he really believes—others’ beliefs proven by behavior more than words—the agent himself often rejects belief ascriptions contradicting the words he tells himself. Those words are usually his opinions: our ordinary unwillingness to attribute an opinion to someone who can’t express it shows that opinions are closely tied to particular words. Opinion is the construct more suitable for possible neurobiological reduction, whereas belief is a family-relations concept, not a sharply delineated entity.