What is an example of logos during Atticus's case in Harper Lee's To Kill a Mockingbird?

The rhetorical appeal referred to as logos, derived from the ancient Greek word logos, has come to refer to an appeal to logic, just as we derive the word logic from logos. In other words, a writer or speaker uses logos to convince an audience of the logic and rationality of an argument. Examples of logos are facts, statistics, and references to authorities--all of which can be used to convince an audience of the logic of an argument. In Harper Lee's To Kill a Mockingbird, any evidence in Atticus's court case, direct or indirect, counts as logos.

One of the first pieces of evidence that calls into question the validity of charging Tom Robinson with the crime is the very fact that neither the Ewells nor Sheriff Heck Tate called a doctor to the scene of the crime the evening the alleged crime took place. We learn this based on the answer to Atticus's very important opening question addressed to Sheriff Tate during Atticus's cross-examination of Tate as a witness: "Did you call a doctor, Sheriff? Did anybody call a doctor?" Atticus rephrases the question multiple times, and each time Sheriff Tate answered in the negative.

The reason why Sheriff Tate's answer to that question is so critical is because it is illegal within the American court system for a person to be charged with a crime if there is no concrete evidence the crime actually took place. Though it was obvious from Mayella's bruises that she had been physically abused by someone, Mayella wasn't just accusing Robinson of physical abuse; she was accusing him of rape. A doctor's evidence and testimony would have been needed to prove she had actually been raped that night in order for Robinson to be legally indicted. We refer to this legal principle as corpus delicti, which translates from the Latin to mean "the body of the crime." We interpret the principle to mean that physical evidence must exist proving a crime was committed before a person can be charged with a crime (West's Encyclopedia of American Law, ed. 2).

Hence, the very fact that no doctor was called, which means that no physical evidence exists, serves as logos that convinces an audience of Robinson's unlawful criminal charge and his innocence. All other similar evidence in the case also serves as logos.