The prima facie principle is a principle that deals with justified belief in cases where opposing evidence is absent. The principle can be stated as this:
P. If one has prima facie evidence for P and lacks evidence for not-P at time t, then one is justified in believing P at time t.
Prima facie evidence is such evidence that in absence of other evidence that evidence is enough to establish a justified belief. Suppose we grant (P), then the question of which evidence is prima facie evidence arises. But a more interesting question is what conditions must prima facie evidence satisfy? I think we ought to define evidence first.
Evidence is a somewhat vague term. Evidence for what exactly? Many dictionaries define evidence as something that makes another thing more likely to be true. So, evidence is relative to a proposition. In Bayesian terms, we can define evidence as:
P(o|e) > P(o)
Evidence is whatever makes some proposition, o, more likely than without taking the evidence into account. When are we justified in believing some proposition? Maybe it is when the probability that it is true is larger to some degree than the probability that it is false.1 Suppose that to be justified in believing a proposition, the probability of that proposition must be at least 0.55 given the available evidence. In that case, prima facie evidence is any evidence that makes the probability of a proposition become 0.55 or greater.
We cannot say that it is evidence with a magnitude of 1.5, because that way we wound assume that the prior probability is always 0.5, but we cannot assume that. The evidential strength of prima facie evidence is, therefore, relative to the prior probability of the proposition.
There are some interesting areas where prima facie evidence is used. One example could be an attempt to justify empiricism as a method of inquiry (broadly speaking). An argumentation could go as this: Empiricism seems to be true, therefore, in absence of any counter evidence (i.e. evidence for the negation) then we are justified in believing empiricism.
The key word is ‘seems’. The idea is that what something seems to be, is prima facie evidence for that it is what it seems to be. Our initial impression of something is based on intuition (among other things), so intuition is a source of prima facie evidence. But how do we know that? Experience has taught us, but then we are begging the question in our justification of empiricism; we assumed that experience is reliable to justified that experience is reliable.
Another application could be an argument for the existence of god. The argument is that, many people know god (or think they know god) and that is prima facie evidence of god. If we suppose that all atheistic evidence has been rebutted, then one is justified in believing in god.
1I suggested this before, see “Et lidt længere forsvar af evidentialisme”