Earlier I have mentioned the fact that when someone holds a position, this very fact is evidence for his position. Here I will consider this in more detail.
The reason to think that the claim is evidence for the position is that it seems more likely that someone would hold a position if the position is true than if it is false. It is evident that this must hold in general, or it would be impossible to learn a language, since people would be equally likely to say “the sky is blue” even if it was not blue, and therefore it would be impossible for children to learn that this sentence says that the sky is blue rather than that the sky is green.
However, someone might object that it is not true in general, and that in some cases claims either have no evidential effect, or that they are evidence that the claim is false.
What would be necessary for this to be true? Let’s take a case where the claim might have no evidential effect at all. Suppose someone says that exactly one year from today, you will eat strawberries for dessert. We might suppose this has no effect: the person has no way of knowing what you will be eating, and therefore he seems equally likely to make the claim, whether you will be eating strawberries or not.
But unless we unreasonably think that it is absolutely certain that prophetic knowledge of the future does not exist, then there is some probability that the statement is prophetic. This will make him somewhat more likely to make the statement if you will in fact be eating strawberries, unless there is a completely equal chance of his statement being anti-prophetic, that is, being made because you will not be eating strawberries. But this would equally require that he know the future, and consequently this amounts to saying that he is equally likely in general to assert or deny the eating of strawberries, even when he knows the truth. But we already admitted that this is not the case: someone who knows the truth is, in general, more likely to assert the truth than to deny it. Thus it is unreasonable to deny that such a statement is in fact evidence that you will eat strawberries for dessert a year from now.
In order for a claim to be evidence that the thing is false, we would have to have something similar: a case where someone who knows the truth is more likely to deny it than to assert it. This would not clearly be the case even, e.g. if we knew that someone was inventing an alibi. It may be that people who invent alibis include more truths than falsehoods in them, taken as a whole. But it could be the case in very concrete circumstances, and taking these circumstances into account. For example, if someone writes a novel “based on a true story,” the fact that the protagonist is called “Peter Smith,” may be evidence that in real life the person’s name was not Peter Smith.
In this case, of course, there is not even a claim that Peter Smith was the person’s name in the first place. So we actually have still not established the existence of such a claim. And if such a case is found, it will be the circumstances, rather than the general fact of the claim, which are evidence against it. Considered in itself, the fact that someone makes a claim or holds a position, is evidence for that claim or position.