Truth lies within a little and certain compass, but error is immense.
Cf. Tolstoy: all happy families are alike, but every unhappy family is unhappy in its own way.
What happens twice probably happens more than twice: are there other notable expressions of this idea?
(There's a well-known principle in software development that's pretty close, though I can't find a Famous Quotation of it right now: when you're choosing a name for a variable or function or whatever, avoid abbreviations: there's only one way to spell a word right, and lots of ways to spell it wrong. Though this is not always good advice.)
Another month, another rationality quotes thread. The rules are: