Epistemology: The Value of Evidence & Propositional Evidence [Part 1]

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Photo Credit: Law Office of Peter Bliar, 2015.

Importance of Evidence

Evidence is important to those of us who wish to come to reasonable conclusions. It is also an important question of epistemology, and thus a significant topic of discussion within the philosophy of science (1). The late British philosopher and logical positivist, A.J. Ayer penned that “I think that if one were looking for a single phrase to capture the stage to which philosophy has progressed, ‘the study of evidence’ would be a better choice than ‘the study of language” (2).

Usually within epistemology, the notion of evidence is relevant to justified belief. Justified belief is often seen as a necessary condition for knowledge. Thus, in many academic disciplines, like science and history, for example, evidence is valuable given that it will ultimately confirm or discredit a scientific or historical theory. Evidence also no doubt further plays a crucial role in law and within our law courts.

The Diversity of Evidence

It is important to note that there are different types of evidence. In the process of a trial, the plaintiff can appeal to bloodstains, fingerprints, and testimony as evidence for someone’s involvement in a crime. And often such strands of data are powerful enough to warrant a verdict to convict an individual of a crime or alternatively release them from accusation. For other types of evidence, perhaps that found within the fields of astronomy or paleontology, observation plays the central role given that such data is not susceptible to experimental manipulation. In the profession of history, evidence commonly comes down to us in historical-narrative documents (fragments, papyrus, scrolls) and via archeology (statues, monuments, coins, and pottery). Within medical science, x-rays, MRI scans, and even subjective experiences of pain, constitute evidence. We even value evidence on more mundane things such as deciding the ripeness of a specific fruit based on its taste and/or appearance.

Now, though the types of evidence we have at our disposal are quite clearly diverse, there are common characteristics that underly all of them. Perhaps most obvious is that evidence either supports or runs contrary to something. The evidence either convicts a defendant as guilty or it does not, it either supports a scientific theory or it does not, it either testifies to the truth of a historical claim or it does not, medical evidence either suggests that somebody is suffering from a certain illness/disease or it rules it out, and so on. Moreover, evidence also comes in range of strengths. An evidence for a specific historical claim, for example, can be argued to constitute better historical evidence than an alternative hypothesis. Evidence can also support improbable facts. For example, a hypothetical case would have a ship-wreck survivor Jack stranded on an island. 25 years slip by and Jack is fairly certain that he is the only human being on the island given the obvious lack of signs of any other human life. All considered, Jack would have sufficient reasons to conclude that he is alone. However, if one day Jack unexpectedly discovered a footprint in the sand he would be rational to conclude that someone else was living on the island. In other words, a single footprint overturned all the evidence to the contrary.

Evidence can also be overturned and undermined in a number of ways. One, it could be opposed by conflicting evidence and/or contradicted by new alternatives. For instance, though circumstantial evidence presented in court might appear convincing in tying the defendant to a specific crime, new evidence might come to light suggesting that the defendant could not have committed the said crime because she was not at the destination in which the crime took place at the time it occurred, thus removing her from culpability. The other evidence then ought to be revised, re-examined, or rejected. Alternatively, opposing evidence might not be sufficient or compelling enough to overturn a currently held position/view. Compelling data might convincingly show that Jill committed the murder, and the single internally inconsistent testimony of one of Jill’s friend saying that she did not commit the crime would not constitute sufficient evidence to overturn the other persuasive evidence suggesting her culpability.

Propositional Evidence

Taken together, the nature of evidence has raised some significant philosophical questions. Consider, for example, the evidence of a bloody knife used in court as a key piece of evidence. The question would then be if such an object could be used as evidence in the first place? And though from the vantage point of modern forensics the answer to that question would be intuitively obvious, there is more to consider.

Why? Because the bloody knife by itself says nothing. It is, after all, simply a knife with blood on it. Thus, a number of philosophers hold to the view that what constitutes evidence are not physical objects themselves (the knife and the blood stain) but rather a set of believed propositions about the objects (3). What we believe about the bloody knife constitutes the evidence, namely, our belief that it was used in the murder (4). Williamson argues that propositional evidence is central in explanatory reasoning by which one holds to a hypothesis that provides the best explanation of the evidence (5). According to Williamson hypotheses explain propositions, thus, what one tries to explain is something true about a knife, such as that it is bloody.

One of the benefits of propositional evidence is that a proposition can either be believed or disbelieved. The proposition that Jim used the knife to murder his friend Philip over a private dispute can either be believed or disbelieved. Viewing evidence as propositional thus helps pave common ground when discussing evidence itself.


1. Kelly, T. 2014. Evidence. Available; DiFate, V. Evidence. Available.

2. Ayer, A. 1982. Philosophy in the Twentieth Century. p. 18.

3. Copi, I., Cohen, C., & McMahon, K. 1953. Introduction to Logic. p. 95.

4. Williamson, T. 2000. Knowledge and its Limits. p. 194-200.

5. Williamson, T. Ibid.

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