Is Gregor Samsa from Kafka's The Metamorphosis insane?
In order to answer this question, we must look at the point of view from which the story was written. The Metamorphosis is written in the third person -- the "he, she, they" style of narration -- and not from the first person -- the "i, me" style of narration. This indicates that the narrator is reliable, and we can take at face value that the facts relayed to us via the narration are true. In fact, right from the first line of the story, we know what kind of world we're in:
When Gregor Samsa woke up one morning from unsettling dreams, he found himself changed into a monstrous vermin.
This sentence is an anacoluthon, which is defined as "syntactical inconsistency or incoherence within a sentence". At the beginning, it is logical that Gregor Samsa would wake in the morning from unsettling dreams, but not that he would find himself changed into an insect. Right away, the expectation for the whole story is established, and we know that we are no longer in a place of strict naturalism. As such, it can be concluded that Gregor Samsa is quite sane, and also that he is definitely a bug.