But And Then There Were None although a traditional mystery, goes a couple steps beyond more mysteries. One the surface, we have a number of the same limitations: an isolated island, ten people, one of whom is a murderer, and the number grows less as the book progresses. We collect more information about each character as we go on, informing us as to the likely candidates... we make our guesses and then we re-guess after our first guesses are done away with.
As I was reading the book, these were the guesses I made, and the reasons (all of this is revealed in the first half of the novel):
-Anthony James Martin has the moral cause to kill-- human life doesn't mean much to him, so if he had enough cause (even possibly for his own entertainment) he might kill folks. Also, he is the first killed, by his own hand, which still makes him a suspect (although a doctor declaring him dead is a strong case against him as a suspect)
-Dr. Armstrong has the means to kill. He had the poisons and the intelligence to arrange the rest of the murders. Although there is no real motive, ease of opportunity is a significant factor.
-Judge Wargrave might very well agree with the motive given by the mysterious voice. He is a judge and so concerned with justice, and the motive was to sentence those who were declared (by the voice) guilty of murder. He is the one who might be most concerned to see the sentence carried out.
Of course, Lombard had a gun (in turn of the century Britain), which makes him suspect. Thomas Rogers might have greater access to the rest of the house and had more time to investigate the island to set the murders up, and he has access to the food and drink. Emily Brent seems judgmental enough to condemn the others and is crazy in the head.
We have plenty of information about each character to make a decision... but this is where the ideals of the "age of reason" fall short. We both have too much and too little information. Every person on the island is already a murderer, so they are all capable of it. We don't know the secret motives of whoever the murderer is. Even in a limited scope-- ten people on an isolated island-- there are too many details for us to discover truth.
Doesn't that teach us something about truth in general? If we are counting on truth being obvious if only we have accumulated enough facts, we can see that even in an extremely limited universe, which is explained to us the simplest, straightforward language, there is still too much for us to discover the important truth of who is taking life. None of the ten can save their lives from the judgment, because no matter how clear headed they are, they cannot distinguish the significant facts from the insignificant. And, of course, because they are in the midst of a life and death situation, they are not as clear-headed as necessary to make clear decisions and analyze correctly.
In a sense, this island is exactly the same circumstance we all find ourselves in. We are all seeking the most significant truth for the sake of our lives, but reality is too complex, and it is too easy to focus on the red herrings, and the very things we think that might saves us, the very deductions we depend on, might end up killing us.
Spoiler ahead: This is the most cynical of novels, where we find out the murderer only when it is too late. No one can be saved. No one can even be brought to justice. The murderer wins, and we only discover the truth because he determines to let us know. Reason, deduction, the entire age of enlightenment is suspect because the truth that was most significant could not be discovered, even in the pages of a simple murder mystery.
Truth is forever hidden from us. Meaning and life must be determined outside of reason or the search for truth.