I'm planning an article about Wikipedia, and while researching it, I found this description of probatio diabolica - a nice evocative term for "a legal requirement to achieve an impossible proof". It occurs to me that that's what I was talking about at the tail end of my most recent Colour commentary - one of my objections to Bill C-12 is that it creates a probatio diabolica when it forbids description of acts that "would be" illegal, because it's not always possible to know whether the acts described in a fictional story would be illegal. To evaluate legality of fictional characters' actions we would essentially have to put the fictional characters on trial, and since they do not exist, and no more is known about them than whatever is written in the story, gathering evidence may be difficult, even before we examine stories like the one I hypothesized, where the story may be constructed in a mathematically self-referential way that defies analysis.
The Wikipedia article suggests two solutions to probatio diabolica; in the first paragraph it says "remedies include", suggesting that other remedies may also exist, but later in the article is seems pretty definite that the two listed remedies are the only ones possible. The two remedies suggested are reversing the burden of proof, so that someone who can do the proof ends up doing it, instead of someone who can't; and giving the person who must do the proof additional rights like "discovery", so that the proof will become possible.
How would we apply these to the child pornography question? Well, reversing the burden of proof seems like a really bad idea to me. In a criminal prosecution it's the prosecutor's job to prove that the accused did something wrong; it would be the prosecutor's job to prove that the characters in the story did something that "would be" illegal. If that is not possible, for instance because the characters' ages were never stated, then it seems like reversing the burden of proof would mean requiring the accused to prove that the characters were of legal age. The accused might be in a better position to do that because of having notes, or a fully formed vision of the characters, going beyond what's in the actual words of the story. Of course, that's assuming the accused is even the author, which is by no means guaranteed. I don't like putting the burden of proof on the accused - that goes against fundamental principles of criminal law - and I'm not sure it would work anyway in the case where the "evidence" needed to prove innocence or guilt exists only in the accused's mind. How are we supposed to know whether a statement like "This story doesn't say how old little Suzy is, but I imagine her as being about 23" is true or not?
Giving the prosecution additional "discovery" rights seems like even more of a non-starter... they already can be assumed to have (or to have had) police powers to conduct searches and so on, and so if they're still in a probatio diabolica situation, I don't know what additional rights could get them out of it. Fictional material like my hypothetical self-referencing story seems to have the probatio diabolica built in; adding additional evidence can never remove the contradiction. The evidence to prove how old the characters are, or whether they consented, etc., may simply not exist to be discovered.
What are some other solutions? The story I described splits naturally into two parts, each of which could be argued to be innocuous and easy-to-analyse by itself. If we could look at the two parts separately and say "this part is legal to possess, that part is legal to possess, so the two of them together are legal to possess" then we wouldn't have the problem. The trouble is, that kind of approach doesn't work, or at least, only works up to a point. Imagine splitting a story up into words and proving that each word is innocuous; if that proved that the whole thing was okay, then there wouldn't be any point having a law regulating text at all. It's easy to construct texts in which any deletion or splitting, no matter how small, will destroy critical meaning. In fact, that's arguably the best definition of what a haiku is supposed to be.
Another thing we could do, and probably the right one, and probably the one that would actually be used in practice, would be to simply recognize the probatio diabolica, prove that it cannot be resolved, and then abandon attempts to resolve it and decide the actual case on the basis of something else. The fact that the law can do that seems to be one of the critical differences between the law and mathematics.