Let me introduce this section, then, by discussing something that couldn't be brought up until we had discussed effect and cause. We are now in a position to define what evidence is:
The evidence for some fact is a known effect whose cause is that fact.(1)
This applies in all cases except, of course, self-evident facts. They really have no evidence, because you can't really wonder whether they are true or not; if you do, it's because you don't understand what they mean, as we saw when we were discussing the self-evident facts that there is something and that what is true cannot be false in the respect in which it is true. Just as nothing can be the cause of itself in the strict sense (because then it is not an effect), so nothing can be its own evidence (because then its truth was not in question in the first place).
Hence, evidence is the "reason" why we know something that is not self-evident. If the fact is not self-evident (known to be true simply by understanding what it means), then it is "evident through something else," which has to mean that you know that Fact A (the one you have evidence for) is true because Fact B (the evidence) is a fact and couldn't be this fact unless Fact A is a fact.
This, of course, is another way of saying that Fact B is an effect of Fact A. Hence, evidence is necessarily an effect of the fact it gives evidence of.
That's obvious. But it says something interesting: Evidence is the cause of our knowledge of the fact it is evidence for. That is, if you take the question, "How do you know that X is true, when you are not directly aware of it (it is not self-evident)?" then the cause of this effect (i.e. our knowledge of what is not self-evident) is the fact that the evidence is an effect whose cause is the fact we know through it.
Say that again? The fact that the evidence is an effect (the evidence as demanding Fact X as its cause) is the fact the cause of the effect which is our knowledge-without-direct-access-to. That is, we have two different effects here: first, the effect which is our knowledge-without-direct-access. This effect is explained by the second effect: the impossibility of the evidence without the fact that it is evidence for.
Thus, let us say that you have never been to Rome, but you know that Rome is a real city, not like Atlantis. How do you know? You never saw the place. That's the effect. The reason you know is the fact that so many people have talked to you about Rome that it's (for practical purposes) impossible for them all to have been lying; and so if Rome didn't exist, then they couldn't have said what they said. That's how you know, because of that impossibility-unless (that effect).
So evidence is a cause precisely because it is an effect of some fact. It is an effect of a fact and a cause of our knowledge of the fact.
I am perhaps spending too much time on this; but I think it is apt to be confusing unless one does spend some time. It shows, however, that effects as effects can be causes (i.e. the fact that something or other is an effect can be the solution to a given problem). As a cause, of course, it's a fact, not an effect.Next
1. I learned that evidence is "the clarity of the thing manifested to the mind," a singularly unhelpful definition, it seems to me. What is the "clarity," and how does it get "manifested to the mind"? The definitions in scholastic philosophy are sometimes less clear than what they are defining. Of course, there's the possibility that you think that this definition is somewhat less than pellucid.