ALL STORIES CARRY A SET OF PREMISES THAT THE WRITER BUILDS and offers the reader. These premises are established within the characters and their settings. Sometimes, the premises are givens that have been established by common expectation, knowledge, or popular culture.
For example, most of us will assume (unless we’ve been told otherwise by the writer) that the full moon is what triggers the transformation of a werewolf from a man to a monster. In a vampire story, we’ll accept that a crucifix may be enough to offer protection, because that’s the popular myth (unless we’re given a reason as to why that doesn’t work in the writer’s tale). If we’re told in yet another story – say, a hard sf piece – that the protagonist is an astrophysicist, we’re going to assume he has a pretty good grasp of astronomy. Logic faults occur when the writer hasn’t given the reader enough information as to why his story world is the way it is, or why it is different. Sometimes the characters of the story don’t act in the way we expect them to (due to common belief), or they know too little or too much.
- A group of NASA scientists make frantic preparations in an effort to deflect a comet about to wreak havoc on the Earth. They have been blindsided by its presence, as they only noticed its appearance the day before. (Unlikely. NASA is vigilant. With all of their equipment and focus on the heavens, it’s unlikely they’d be surprised by a rogue comet. More likely, they'd have been aware of its presence for months.)
- An angel works in the slummy side of heaven because she’s not that enthusiastic about being an angel. Her job as a garbage collector makes her even more depressed, despite being told by her superiors that her efforts are worthwhile. (The popular view is that heaven is a perfect place and that angels are perfect beings. So why does this heaven have slums? Either the protagonist isn’t really an angel, or she’s in purgatory or hell. In any event, more explanation is needed.)
- A sorcerer summons Death who shows up as a possessed raven. After failing to answer Death/Raven’s riddle correctly, the sorcerer dies. Death slips from the bird and exchanges riddles with the raven, who wins the contest. (The strange plot line is confusing. Why does Death become a raven in the first place? Is this from a belief system with which many of us may not be familiar? Why does the raven not remember being possessed by Death? Does Death die because he loses the contest? If so, what happens to him? And finally, something which has nothing to do with logic or the lack thereof, but has everything to do with story - why tell me this tale? What is the point? How does it entertain? Too often, aspiring writers forget about the reader in the writer/reader equation.)
- A domestic robot investigates a virus infestation of the more highly advanced robots in her factory. She realizes that their minds are being sapped for energy, in a similar way that vampires leech their victims of blood. (How is it that a domestic ‘bot’ can investigate anything other than cleaning floors and toilets? How does it know anything about viruses or vampires? Why would a domestic bot have this information stored in its memory banks?)
- Because he is squeamish and hates germs, a medieval strong man loses a boxing match to his opponent, who has a very bad cold. (No. Germs were unknown in the middle ages. How is it that the strong man knows about them and links them to his opponent’s condition? This logic fault is also a type of anachronism which I've discussed before under A is for... Anachronisms)