There is no such thing as evidence – at least not if any genuine “evidence” would have to meet both of these conditions:
- It is your evidence that justifies all of your beliefs or credences that are justified at all.
- Evidence is at least roughly similar to what is called “evidence” in everyday English.
As I shall argue, there is nothing that meets both these conditions. There is indeed something that justifies all of your beliefs or credences that are justified at all, but it is radically unlike what is called “evidence” in ordinary English. Philosophers who assume that your evidence is what justifies your beliefs are in constant peril of being led astray by their linguistic intuitions about the meaning of ‘evidence’ in everyday English.
My conclusion will be that the word ‘evidence’ should be banished from fundamental epistemology altogether.
A. First, let me suggest an account of how the term ‘evidence’ is used in everyday English. (Incidentally, no other language known to me has any word that corresponds closely to ‘evidence’: the French word évidence means either the property of being evident or an evident truth; German also has no closely corresponding word. “Evidence” is a much less universal concept than some philosophers suppose!)
In English, we typically use the word ‘evidence’ in conversations that aim to reach an agreed answer to a question Q. (E.g., Q might be ‘Is theory T true?’ or ‘Did the defendant commit the crime?’ or the like.) We use the “evidence” as our basis for trying to answer this question Q – where every part of what we use as our basis for answering Q is something that we are (in that context) presupposing to be a fact. We accept the claim that p is part of the evidence only if it seems to make sense to presuppose p to be a fact, for the purposes of that particular conversation.
In short, I propose, in any context C, it is true in C to say that p is part of the “evidence” if and only if p is a proposition that it makes sense to presuppose in C, given the purpose of the conversation to try to reach an agreed answer to the relevant question.
According to this account, the term ‘evidence’ is acutely context-sensitive. It usually makes no sense to presuppose a proposition that is controversial among the participants in the conversation; so what counts as part of the “evidence” usually depends on who we are talking to. The proposition that the earth is over 4 billion years old may be part of the “evidence” when we are talking to our scientific colleagues, but not when we are talking to religious fundamentalists.
B. Thus, our linguistic intuitions lead us to assume that our “evidence” consists of propositions that (i) we presuppose or believe, and (ii) are uncontroversial – part of a public “common ground”.
In my view, this explains why it seems plausible to so many Bayesian epistemologists that one’s “evidence” consists of propositions in which one has the highest possible credence, and why it seems plausible to Williamson that one’s “evidence” consists of the propositions that one knows. It also explains why so much of the literature on disagreement assumes that it is common for the parties to a disagreement to have shared evidence.
But in fact, most epistemologists do not accept that the only thing that can make your beliefs rational or justified are propositions that you believe. Most epistemologists would insist that a much wider range of mental states and mental events – including sensory experiences, sensations, memories, intuitions, intrinsically conditional beliefs, desires, emotions, plans, intentions, and the like. In general, your beliefs are not just based on your other beliefs. There are many complex and diverse processes that can lead you to form, revise, and maintain our beliefs.
It seems plausible that all of these mental states, events, and processes can be part of what makes your beliefs rational or justified. But these states, events and processes are not propositions that you believe, and they are also not public or uncontroversial or part of the shared common ground. It is in fact impossible for anyone else to share these mental states or events or processes.
So the claim that your “evidence” is what justifies your beliefs is grotesquely implausible, if the term ‘evidence’ is used in anything like its ordinary sense.
For this reason, the use of the term ‘evidence’ in much epistemology is a constant source of erroneous assumptions. We would be much better off if we eliminated the term ‘evidence’ from fundamental epistemology entirely.