evidence

Philosophy of evidence

Characteristics

Understood in its broadest sense, evidence for a proposition is what supports this proposition. Traditionally, the term is sometimes understood in a narrower sense: as the intuitive knowledge of facts that are considered indubitable. In this sense, only the singular form is used. This meaning is found especially in phenomenology, in which evidence is elevated to one of the basic principles of philosophy, giving philosophy the ultimate justifications that are supposed to turn it into a rigorous science. In a more modern usage, the plural form is also used. In academic discourse, evidence plays a central role in epistemology and in the philosophy of science. Reference to evidence is made in many different fields, like in science, in the legal system, in history, in journalism and in everyday discourse. A variety of different attempts have been made to conceptualize the nature of evidence. These attempts often proceed by starting with intuitions from one field or in relation to one theoretical role played by evidence and go on to generalize these intuitions, leading to a universal definition of evidence.

One important intuition is that evidence is what justifies beliefs. This line of thought is usually followed in epistemology and tends to explain evidence in terms of private mental states, for example, as experiences, other beliefs or knowledge. This is closely related to the idea that how rational someone is, is determined by how they respond to evidence. Another intuition, which is more dominant in the philosophy of science, focuses on evidence as that which confirms scientific hypotheses and arbitrates between competing theories. On this view, it is essential that evidence is public so that different scientists can share the same evidence. This leaves publicly observable phenomena like physical objects and events as the best candidates for evidence, unlike private mental states. One problem with these approaches is that the resulting definitions of evidence, both within a field and between fields, vary a lot and are incompatible with each other. For example, it is not clear what a bloody knife and a perceptual experience have in common when both are treated as evidence in different disciplines. This suggests that there is no unitary concept corresponding to the different theoretical roles ascribed to evidence, i.e. that we do not always mean the same thing when we talk of evidence.

On the other hand, Aristotle, phenomenologists, and numerous scholars accept that there could be several degrees of evidence. For instance, while the outcome of a complex equation may become more or less evident to a mathematician after hours of deduction, yet with little doubts about it, a simpler formula would appear more evident to them.

Evidential relation

Philosophers in the 20th century started to investigate the "evidential relation", the relation between evidence and the proposition supported by it. The issue of the nature of the evidential relation concerns the question of what this relation has to be like in order for one thing to justify a belief or to confirm a hypothesis. Important theories in this field include the probabilistic approach, hypothetico-deductivism and the positive-instance approach.

Probabilistic approaches, also referred to as Bayesian confirmation theory, explain the evidential relation in terms of probabilities. They hold that all that is necessary is that the existence of the evidence increases the likelihood that the hypothesis is true. This can be expressed mathematically as

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{\displaystyle P(H\mid E)>P(H)}

. In words: a piece of evidence (E) confirms a hypothesis (H) if the conditional probability of this hypothesis relative to the evidence is higher than the unconditional probability of the hypothesis by itself. Smoke (E), for example, is evidence that there is a fire (H), because the two usually occur together, which is why the likelihood of fire given that there is smoke is higher than the likelihood of fire by itself. On this view, evidence is akin to an indicator or a symptom of the truth of the hypothesis. Against this approach, it has been argued that it is too liberal because it allows accidental generalizations as evidence. Finding a nickel in one's pocket, for example, raises the probability of the hypothesis that "All the coins in my pockets are nickels". But, according to Alvin Goldman, it should not be considered evidence for this hypothesis since there is no lawful connection between this one nickel and the other coins in the pocket.

In specific fields

Important theorists of evidence include Bertrand Russell, Willard Van Orman Quine, the logical positivists, Timothy Williamson, Earl Conee and Richard Feldman. Russell, Quine and the logical positivists belong to the empiricist tradition and hold that evidence consists in sense data, stimulation of one's sensory receptors and observation statements, respectively. According to Williamson, all and only knowledge constitute evidence. Conee and Feldman hold that only one's current mental states should be considered evidence.

The guiding intuition within epistemology concerning the role of evidence is that it is what justifies beliefs. For example, Phoebe's auditory experience of the music justifies her belief that the speakers are on. Evidence has to be possessed by the believer in order to play this role. So Phoebe's own experiences can justify her own beliefs but not someone else's beliefs. Some philosophers hold that evidence possession is restricted to conscious mental states, for example, to sense data. This view has the implausible consequence that many of simple everyday-beliefs would be unjustified. The more common view is that all kinds of mental states, including stored beliefs that are currently unconscious, can act as evidence. It is sometimes argued that the possession of a mental state capable of justifying another is not sufficient for the justification to happen. The idea behind this line of thought is that justified belief has to be connected to or grounded in the mental state acting as its evidence. So Phoebe's belief that the speakers are on is not justified by her auditory experience if the belief is not based in this experience. This would be the case, for example, if Phoebe has both the experience and the belief but is unaware of the fact that the music is produced by the speakers.

It is sometimes held that only propositional mental states can play this role, a position known as "propositionalism". A mental state is propositional if it is an attitude directed at a propositional content. Such attitudes are usually expressed by verbs like "believe" together with a that-clause, as in "Robert believes that the corner shop sells milk". Such a view denies that sensory impressions can act as evidence. This is often held as an argument against this view since sensory impressions are commonly treated as evidence. Propositionalism is sometimes combined with the view that only attitudes to true propositions can count as evidence. On this view, the belief that the corner shop sells milk only constitutes evidence for the belief that the corner shop sells dairy products if the corner shop actually sells milk. Against this position, it has been argued that evidence can be misleading but still count as evidence.

Different types of evidence

In science (empirical evidence)

In scientific research evidence is accumulated through observations of phenomena that occur in the natural world, or which are created as experiments in a laboratory or other controlled conditions. Scientists tend to focus on how the data used during statistical inference are generated. Scientific evidence usually goes towards supporting or rejecting a hypothesis.

The burden of proof is on the person making a contentious claim. Within science, this translates to the burden resting on presenters of a paper, in which the presenters argue for their specific findings. This paper is placed before a panel of judges where the presenter must defend the thesis against all challenges.

When evidence is contradictory to predicted expectations, the evidence and the ways of making it are often closely scrutinized (see experimenter's regress) and only at the end of this process is the hypothesis rejected: this can be referred to as 'refutation of the hypothesis'. The rules for evidence used by science are collected systematically in an attempt to avoid the bias inherent to anecdotal evidence.

In law

In law, the production and presentation of evidence depend first on establishing on whom the burden of proof lies. Admissible evidence is that which a court receives and considers for the purposes of deciding a particular case. Two primary burden-of-proof considerations exist in law. The first is on whom the burden rests. In many, especially Western, courts, the burden of proof is placed on the prosecution in criminal cases and the plaintiff in civil cases. The second consideration is the degree of certitude proof must reach, depending on both the quantity and quality of evidence. These degrees are different for criminal and civil cases, the former requiring evidence beyond a reasonable doubt, the latter considering only which side has the preponderance of evidence, or whether the proposition is more likely true or false.

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. The decision-maker, often a jury, but sometimes a judge decides whether the burden of proof has been fulfilled. After deciding who will carry the burden of proof, the evidence is first gathered and then presented before the court:

In a criminal investigation, rather than attempting to prove an abstract or hypothetical point, the evidence gatherers attempt to determine who is responsible for a criminal act. The focus of criminal evidence is to connect physical evidence and reports of witnesses to a specific person.

The path that physical evidence takes from the scene of a crime or the arrest of a suspect to the courtroom is called the chain of custody. In a criminal case, this path must be clearly documented or attested to by those who handled the evidence. If the chain of evidence is broken, a defendant may be able to persuade the judge to declare the evidence inadmissible.

Presenting evidence before the court differs from the gathering of evidence in important ways. Gathering evidence may take many forms; presenting evidence that tends to prove or disprove the point at issue is strictly governed by rules. Failure to follow these rules leads to any number of consequences. In law, certain policies allow (or require) evidence to be excluded from consideration based either on indicia relating to reliability, or broader social concerns. Testimony (which tells) and exhibits (which show) are the two main categories of evidence presented at a trial or hearing. In the United States, evidence in federal court is admitted or excluded under the Federal Rules of Evidence.