Dr. Bryant recommends that his students imagine growing a second brain, one whose role is to double-check the assumptions of one’s primary brain. The second brain is there to challenge how conclusions were reached, to ask “what form of reasoning is this?” and “How do I know that if a then b must be true?” This is a point that Maria Konnikova makes in her book: that we would do well to question what we hear immediately, rather than absorbing it as fact and only then questioning it in reflection. By way of example, consider hearing someone mention a “pink elephant.” We instantly imagine an elephant with pink skin, before we “engage in disbelieving it.” Konnikova writes “Holmes’ trick is to treat every thought, every experience, and every perception the way he would a pink elephant…begin with a healthy dose of skepticism instead of credulity…” The implication is that our instinctive credulity puts us at a disadvantage—when we accept what is said (an eyewitness account of a crime, for example), we absorb the prejudice of at first believing what we hear, and only later considering how we might change our mind. Skip this initial step of believing what you hear, and you’ll think more clearly.

That sounds good on paper, but Sherlock Holmes did not regularly follow the advice that Maria Konnikova teaches based on Holmesian examples. In the instance of the dog in the night, Holmes solved the crime, but did so with flawed logic.

Sherlock Holmes should have asked himself why he concluded that a dog not barking meant that the race horse thief was someone the dog knew. Holmes did not consider alternative explanation for why the dog might not have barked, just like Holmes might have assumed that the solution to Moser’s Circle Problem is 32, and have therefore gotten it wrong.