Evidence for a proposition is what supports this proposition. It is usually understood as an indication that the supported proposition is true. What role evidence plays and how it is conceived varies from field to field. In epistemology, evidence is what justifies beliefs or what makes it rational to hold a certain doxastic attitude. For example, a perceptual experience of a tree may act as evidence that justifies the belief that there is a tree. In this role, evidence is usually understood as a private mental state. Important topics in this field include the questions of what the nature of these mental states is, for example, whether they have to be propositional, and whether misleading mental states can still qualify as evidence. Other fields, including the sciences and the legal system, tend to emphasize more the public nature of evidence. In philosophy of science, evidence is understood as that which confirms or disconfirms scientific hypotheses. Measurements of Mercury's "anomalous" orbit, for example, are seen as evidence that confirms Einstein's theory of general relativity. In order to play the role of neutral arbiter between competing theories, it is important that scientific evidence is public and uncontroversial, like observable physical objects or events, so that the proponents of the different theories can agree on what the evidence is. This is ensured by following the scientific method and tends to lead to an emerging scientific consensus through the gradual accumulation of evidence. Two issues for the scientific conception of evidence are the problem of underdetermination, i.e. that the available evidence may support competing theories equally well, and theory-ladenness, i.e. that what some scientists consider the evidence to be may already involve various theoretical assumptions not shared by other scientists. It is often held that there are two kinds of evidence: intellectual evidence or what is self-evident and empirical evidence or evidence accessible through the senses.
In order for something to act as evidence for a hypothesis, it has to stand in the right relation to it, referred to as the "evidential relation". There are competing theories about what this relation has to be like. Probabilistic approaches hold that something counts as evidence if it increases the probability of the supported hypothesis. According to hypothetico-deductivism, evidence consists in observational consequences of the hypothesis. The positive-instance approach states that an observation sentence is evidence for a universal hypothesis if the sentence describes a positive instance of this hypothesis. The evidential relation can occur in various degrees of strength. These degrees range from direct proof of the truth of a hypothesis to weak evidence that is merely consistent with the hypothesis but does not rule out other, competing hypotheses, as in circumstantial evidence.
In law, rules of evidence govern the types of evidence that are admissible in a legal proceeding. Types of legal evidence include testimony, documentary evidence, and physical evidence. The parts of a legal case that are not in controversy are known, in general, as the "facts of the case." Beyond any facts that are undisputed, a judge or jury is usually tasked with being a trier of fact for the other issues of a case. Evidence and rules are used to decide questions of fact that are disputed, some of which may be determined by the legal burden of proof relevant to the case. Evidence in certain cases (e.g. capital crimes) must be more compelling than in other situations (e.g. minor civil disputes), which drastically affects the quality and quantity of evidence necessary to decide a case.