Epistemology is the branch of philosophy that examines knowledge—its nature, sources, and limits. The classical definition holds that knowledge is justified true belief: a person knows a proposition when they believe it, the belief is true, and they have adequate justification for it. The Gettier problem, however, presents cases in which someone has justified true belief yet seems to lack knowledge, suggesting that justification and truth may not be sufficient. Epistemology also contrasts two major sources of knowledge. Empiricism holds that knowledge comes primarily from sensory experience, while rationalism argues that reason is the principal source, especially for necessary truths. Skepticism pushes these questions further by doubting claims to knowledge whenever sufficient evidence is lacking, demanding rigorous standards for any assertion.
A foundational distinction in critical thinking is between facts, opinions, and value judgments. A fact is a statement that can be verified as true. An opinion is a personal judgment or belief that need not be verifiable. A value judgment is an assessment rooted in personal or cultural values. The is-ought distinction, articulated by Hume, warns that one cannot logically derive what ought to be from what is; the naturalistic fallacy is the error of inferring moral claims from natural facts, while the moralistic fallacy is its reverse, inferring facts from moral preferences. These distinctions help separate descriptive claims about how things are from normative claims about how they ought to be.
Ethics itself can be approached through several lenses. Utilitarian reasoning judges actions by their consequences for overall well-being. Deontological reasoning judges actions by adherence to rules or duties, regardless of outcomes. Virtue ethics focuses on the character traits that make for a good life. Thought experiments such as the trolley problem probe the tensions between these approaches, while the prisoner's dilemma, a classic scenario from game theory, illustrates how individually rational choices can produce collectively worse outcomes. Practical wisdom, called phronesis, is the intellectual virtue of knowing how to act well in particular situations—balancing principles, context, and consequence.
Philosophers have also proposed different accounts of how beliefs are justified. Coherentism holds that a belief is justified by its coherence with other beliefs, forming a web of mutual support. Foundationalism argues that some beliefs are basic and self-justifying, providing the ground on which other beliefs rest. The Münchhausen trilemma observes that attempts to justify beliefs ultimately face three options: circularity, infinite regress, or an arbitrary stopping point. The principle of sufficient reason asserts that everything must have a reason or cause, while pragmatism treats truth as bound up with practical consequences. Teleological reasoning explains things by their ends or purposes, and reductionism, holism, emergence, and supervenience offer competing accounts of how complex wholes relate to their parts. Rationality means acting in accordance with reason and evidence, though bounded rationality, as Simon emphasized, recognizes that human reasoning is limited by available information, cognitive capacity, and time.