Everyone on This Train Is a Suspect

There must be but one detective—that is, but one protagonist of deduction—one deus ex machina. To bring the minds of three or four, or sometimes a gang of detectives to bear on a problem, is not only to disperse the interest and break the direct thread of logic, but to take an unfair advantage of the reader. If there is more than one detective the reader doesn’t know who his co-deductor is. It’s like making the reader run a race with a relay team.

Rule 9, S.S. Van Dine’s “Twenty Rules for Writing Detective Stories,” 1928

A sequel is an admission that you’ve been reduced to imitating yourself.

Don Marquis