Dr Thompson

Dr Thompson is a middle-aged psychiatrist drawn into the investigation of the A B C murders as an expert on the possible mental state of the killer and of Alexander Bonaparte Cust. He offers the police a way to think about motive beyond the visible alphabetical pattern, but his later assessment of Cust remains cautious: epilepsy might permit action without awareness in some circumstances, yet the advance-warning letters make that explanation difficult to accept.

Identity and Introduction

Dr Thompson attends a major police conference the morning after a letter connected with the murders is discussed. Among those assembled are senior Sussex and Criminal Investigation Department officials, Inspector Glen from Andover, Chief Inspector Japp, Inspector Crome, and Dr Thompson, identified as a famous alienist. Though learned, he prefers homely language to professional technicalities. This manner makes his expertise accessible in a room where practical policing, public alarm, and uncertain psychological interpretation meet.

His role is not to identify a suspect from physical evidence. Instead, he helps the investigators ask what kind of motive could account for the apparent selection of victims. The letters and the murders pose a problem of interpretation: if the sequence is genuinely alphabetical, does that explain the killer’s purpose, or merely the arrangement he has chosen to display?

Reading the Pattern and the Motive

Dr Thompson considers Poirot’s suggestion that an alphabetical complex might govern both the place and the victim. The name Ascher had been plainly displayed above the shop of the first victim, while the next letter named Bexhill; the possibility that the next victim’s surname would begin with B therefore seemed worth considering. Thompson treats this as a hypothesis rather than a conclusion. The first two victims might instead share some other feature, such as being women, and the evidence does not yet establish a motive.

He explains why an apparently arbitrary series cannot simply be treated as random violence. Drawing on a case in which a woman killed members of the jury who had condemned her husband, he describes how crimes may seem disconnected until investigators recognize the grievance connecting them. In his account, a murderer generally removes people believed to obstruct him, or kills according to a conviction about who ought to be removed. Mrs Ascher and Betty Barnard do not appear to belong to a common social class, so neither explanation readily fits. A sex-related motive remains possible, but he says more could be learned only after another crime.

That qualification exposes a tension between clinical analysis and the immediate demands of prevention. When Thompson speaks of a possible next crime, Sir Lionel objects that the police intend to prevent one. Thompson does not press the point. The exchange places his pattern-seeking in an ethically uncomfortable setting: the forecast may help investigators anticipate the killer, but each further confirmation would come at a human cost.

The Killer’s Need for Recognition

Thompson’s psychological questions intersect with Poirot’s account of a killer seeking public attention. Poirot imagines a person whose personality has been suppressed, whose sense of injustice and inferiority has accumulated, and whose urge to assert himself has been repeatedly checked. The alphabetical murders and their letters can thus be read not only as a method of selection but as a conspicuous bid to make the killer visible. Thompson is present as the expert interlocutor in this discussion, though the detailed portrait is Poirot’s hypothesis rather than Thompson’s independent diagnosis.

The contrast matters to the inquiry. A killer who acts from private irritation, a fixed conviction, or a wish to advertise himself presents a different problem from one who chooses victims for a personal connection. Thompson helps keep those possibilities open without claiming that the available evidence has resolved them. His provisional approach resists turning an evocative psychological account into proof.

Cust, Epilepsy, and Responsibility

After Cust’s police-court proceedings, Thompson is again asked for his view. Cust has been committed for trial, and the discussion turns to whether his epilepsy could mean that he committed the crimes without knowing it. Thompson acknowledges that an epileptic person in a state of somnambulism might perform an action without awareness. He also gives an important qualification: such an action would generally have to be consistent with the person’s will while awake.

This clinical possibility does not settle the case. Thompson rejects the theory that Cust committed the murders unknowingly, chiefly because of the letters: they indicate premeditation and careful planning. His position therefore distinguishes a general account of what might occur during a disorder from what the evidence in this particular case supports. Cust’s own history lends the question weight: he says he had long experienced lapses, and that a slight head wound during the war led others to discover his fits and discharge him from service. But those circumstances alone do not establish that he carried out the murders or that he was unaware of doing so.

Thompson is not wholly certain about Cust’s present performance, either. He remarks that Cust is playing the sane man remarkably well, while also recognizing him as an epileptic. At the police-court discussion, others disagree: Japp considers the possibility of an alibi weakening the Bexhill case, while Crome believes Cust knows he committed the murders. The competing readings show the limits of Thompson’s contribution: expertise supplies distinctions and possibilities, but it does not remove the need to test them against the letters, movements, and other evidence.

The Stalking Horse and the Limits of Diagnosis

Poirot’s eventual explanation reframes the apparent psychological puzzle. He says that Cust believes himself guilty, yet could not be the real planner: the actual murderer was cunning and resourceful, while Cust was stupid, vacillating, and suggestible. Cust’s lapses, nervousness, and presence near the earlier crimes made him susceptible to the idea that fate had linked him to the series; the true killer had used him as a stalking horse. This revelation clarifies why a diagnosis of epilepsy cannot, by itself, identify the murderer or explain every apparent sign of guilt.

Thompson’s presence in the investigation gives the story a measured vocabulary for uncertainty, but the decisive distinction comes from comparing clinical interpretation with the scheme’s practical evidence. His account of possible unconscious action is neither a verdict nor an exoneration. The letters’ planning weighs against it, while Poirot’s discovery of deliberate manipulation shows how Cust’s condition and suggestibility could be exploited without making him the mastermind. In that contrast, the character’s narrative function is to make the boundary between illness, awareness, and criminal responsibility an explicit question rather than an easy answer.