A second important aspect of epistemic rationality involves the relation between our beliefs and the evidence we have. Intuitively, a belief is reasonable when it fits the evidence well; and a belief is unreasonable when it conflicts with the (total) evidence we have. So, in order to understand what it means to be epistemically rational, we need to understand what it means to have evidence.
When philosophers use the word evidence, we are often speaking of propositions. In the previous chapter, we saw how an evidential proposition E can support a hypothesis H according to Bayesian confirmation theory. Bayesianism provides a useful definition of propositional evidence, to which we’ll turn in a moment.
Not all evidence is propositional, of course. In colloquial usage, the word “evidence” often refers to physical objects of the sort that might be presented in a courtroom: fingerprints, a DNA sample, surveillance video footage, etc. These objects are not propositions, obviously, yet they are considered to be evidence. Similarly, sensations and memories provide non-propositional evidence. For example, the aroma of fresh apple pie may provide evidence that there is a pastry in the oven, and your memories of previous experiences may provide evidence about the flavors you will enjoy when the pie is ready.
Fortunately, in most (perhaps all) cases, we can convert these non-propositional forms of evidence into propositional evidence, simply by identifying a relevant proposition about the objects, sensations, or memories in question. For example, the proposition that the suspect’s fingerprints were found at the crime scene can be regarded as evidence in place of the physical fingerprints themselves. Likewise, the proposition that you smell the aroma of apple pie can be regarded as evidence instead of the aroma itself. This allows us to treat nearly all forms of evidence in a unified way, using the same probabilistic models as conceptual tools to understand how evidence works in general. Three such models of evidence will be introduced in the next section of this chapter.