"Circumstantial evidence is occasionally very convincing, as when you find a trout in the milk, to quote Thoreau's example"

This quote by Arthur Conan Doyle is referring to circumstantial evidence, which is proof that indicates something is true, yet does not straight confirm it. Doyle is saying that circumstantial evidence can be extremely convincing, and also he makes use of the instance of locating a trout in the milk, which was originally used by Henry David Thoreau. This instance suggests that a person has done something wrong, as it is highly not likely that a trout would certainly end up in the milk by accident. For that reason, circumstantial evidence can be really powerful in persuading people of something, also if it does not directly show it. Doyle's quote is a reminder that circumstantial evidence must not be forgotten, as it can be really convincing.