The term ‘evidence’ as used in ordinary discourse has at least two meanings that are relevant for our purposes. The best way to understand these meanings is in terms of the ways they are used in various domains.

In the discourse of the law court, for instance, evidence refers to witness testimonies, records, documents or objects that lawyers present to the jury as the basis of their argumentation. The following examples may serve to connect the concept of evidence in law to evidence in some of the other domains:

• History: written records as evidence for a historical event.
  

• Evolutionary biology: fossils as evidence for the existence of an extinct species.
  

• Medicine: an x-ray picture as evidence of a physical condition.
  

• Chemistry: the results of a chemical experiment as evidence for a scientific theory.
  

• Sociology: the numbers of a survey as evidence for a correlation,
  

• Literary criticism: the words in a poem as evidence for a particular interpretation of the poem.

In the above view of evidence, evidence is viewed as the perceptual grounds that form the basis for the justification of a claim. In many types of justification, such grounds must be accompanied by the reasoning that connects the evidence to the conclusions. [See Ingredients of Critical Thinking for details.]. Now, there is another meaning of ‘evidence’ under which evidence includes not just the perceptual grounds, but any grounds, and it includes reasoning as well. These two of meanings of the term ‘evidence’ can be articulated as:

1. Evidence as the perceptual grounds (‘data’) on the basis of which one provides reasons for or against a conclusion.
   

2. Evidence as the reasons (including both grounds and reasoning) that one provides in support of or against a conclusion.

If we accept the characterization of the meaning of the term in (A), then not all grounds of justification constitute evidence. However, if we accept the meaning in (B), then evidence and justification are synonymous. Take, for example, the proof of a theorem in mathematics. The grounds for a mathematical theorem are the axioms and definitions of postulated in the theory: these are not instances of our perceptual experience: mathematicians do not prove Pythagoras’ theorem by measuring the sides of a large sample of right angled triangles, or the four colour theorem by generalizing from a large sample of maps. Mathematical proofs, therefore, do not require evidence in the sense in (A). However, they do involve evidence in sense (B).

Similar observations can be made about most forms of philosophical argumentation as well. When providing justification for a theory of truth or a theory of knowledge, philosophers do not need to gather data as their grounds. The grounds for justification in such cases are either axiomatic commitments (as in the case of the moral axiom that it is immoral to cause harm.) or the results of introspection (as in the case of thought experiments.). Hence, we may say that philosophical inquiry does not require evidence in sense (A), but it does require evidence in sense (B).

Now, we already have the term ‘justification’ to refer to the reasons for/against a conclusion. Hence, we will restrict the use of the term evidence to (A), to refer to what one cites as the grounds for or against a conclusion.

On the basis of the above our remarks about evidence in scientific, mathematical and philosophical inquiries, we may distinguish the following types of justification on the basis of what is chosen as the grounds:

• Justification based on evidence: history, literary interpretation, biology, physics, …)
  

• Justification based on axioms and definitions: (mathematics, formal logic)
  

• Justification based on axiomatic commitments and introspection: philosophy

Contributed by K P Mohanan ().

See also: justification, proof, grounds