RUTH BARCAN MARCUS AND MODALITY (9): Belief and Reason

Marcus now asks: what does it mean to believe something rationally? How should we understand the structure of belief? And what role does logic play in shaping what we may reasonably believe? These questions might seem simple at first, but they connect directly to the deeper issues of necessity, identity, moral judgment, and the unity of reason that she has been developing throughout the book.

Marcus begins with a simple insight. When someone believes something, they are taking a stand on how they think the world is. Belief is a commitment of the mind. If you believe that whales are mammals or that your friend is sexy or that lying is wrong, you are placing these ideas into the structure of your thinking. She says that belief always has a shape, even when we are not fully aware of that shape. She then stresses that they interconnect. If you believe that whales are mammals, and you also believe that all mammals breathe air, then you are rationally committed to believing that whales breathe air. Logical connections between statements generate rational commitments between beliefs. This is part of the unity of reason. Your belief system is like a web where pulling one thread affects the others.

She explains that rational belief requires coherence. This means your beliefs should fit together without contradiction. If you believe both that “all swans are white” and that “there is a black swan,” then your beliefs clash. Something must change. Marcus says that rationality does not demand that your beliefs are always true, but it does demand that they are consistent with each other. A person can be wrong yet still be rational, as long as they treat their beliefs carefully and revise them when they conflict. She argues that beliefs are not only connected through logic. They are also shaped by evidence and experience. Marcus warns against a view sometimes found in strict formal logic which suggests that rational belief is simply a matter of logical deduction. She says this view is too narrow. People form beliefs by listening, observing, reflecting, and learning. Logic helps organise these beliefs, but it does not generate them out of nothing. Evidence gathers the raw material. Logic shapes the structure.

At the same time, she argues that logical relations among beliefs are real and important. So belief is not just a matter of psychological feeling or social habit. She thinks that logical relations are built into the very idea of having a belief. If you believe A, and you believe that A implies B, then believing B is not optional. You have a rational commitment to it. This commitment arises from the nature of belief itself. She explains how belief interacts with modal reasoning. We've already seen how she analysed statements like “John believes that p.” These are intensional contexts, meaning we cannot freely substitute equivalent expressions into them. Marcus says that belief is sensitive to how a statement is presented. If John believes that “Superman can fly,” he may not know that Superman is Clark Kent. So he may not believe that “Clark Kent can fly.” Rational belief depends not only on truth but also on a person’s grasp of the content. This is why belief contexts resist the simple substitution rules used in extensional logic. This leads to another idea. Rationality in belief is both formal and personal. It has a formal side, which concerns logical connections between statements. It also has a personal side, which concerns a person’s understanding, experience, and evidence. Marcus says a good theory of belief must respect both. A theory that focuses only on logical structure ignores how people actually learn and understand things. A theory that focuses only on psychological facts ignores the logical commitments built into belief.

She then asks whether a person can have inconsistent beliefs and still count as rational? Marcus says people do sometimes fall into inconsistency without noticing, but acknowledging this fact does not mean inconsistency is acceptable. Rationality requires that people are prepared to revise their beliefs when contradictions come to light. The ideal of coherent belief remains important, not as a rigid rule, but as a guiding standard. She also connects belief to action. A rational person does not only hold beliefs. They act on them. If you believe the ice is thin, you avoid stepping on it. If you believe your friend is waiting for you, you hurry to meet them. Marcus says that belief guides action in a structured way. This shows again that belief is part of the unified activity of reason. It is not an isolated mental state. It is connected to decision making, planning, and moral reflection. She also stresses that beliefs can shape what a person thinks is possible. This connects with her earlier discussion of moral commitment. If you believe that honesty is essential, you may not even see lying as a real option for yourself. Belief and commitment interact to shape your practical reasoning. This is why she thinks reason itself is unified. Belief, moral commitment, and practical choice are all aspects of one rational life.

Marcus considers the idea that belief may sometimes require courage. This does not mean faith without evidence. It means the courage to revise your beliefs when new evidence or arguments show you that you were wrong. She says rationality includes a readiness to change one’s mind. It is sensitivity to reasons.

So reason for Marcus is the structure that allows a person to make sense of the world, choose actions, and live according to their values. Belief is one of the main threads in this structure. It shapes how we see, how we judge, and how we act. To understand belief is therefore to understand a major part of reason itself.