Category: consciousness
Type: argument
Associated thinker(s): Thomas Nagel
Original publication or approximate period: Nagel, “Brain Bisection and the Unity of Consciousness” (Synthese, 1971)
Difficulty: Intermediate
Themes: unity of consciousness, split brain, number of minds
Related experiments: Fission and Personal Identity; What Is It Like to Be a Bat?; Brain Transplant: Brown and Robinson
Some people with severe, medication-resistant epilepsy have undergone partial or complete surgery to sever the corpus callosum. Several classic operations also severed other cerebral commissures. In controlled tests, information flashed to one visual field may be processed primarily by the opposite hemisphere and available to the hand it predominantly controls, even when speech or the other hand does not reveal the same information. The exact pattern depends on the patient, the extent of disconnection, the task, and the response being measured. See the primary reports by Gazzaniga, Bogen, and Sperry (1962) and Sperry (1968).
Patients can show little obvious disruption in ordinary social behavior. But those everyday observations are largely anecdotal; they do not establish that patients are unaffected in daily life. A 2020 review summarizes both evidence for disconnection and evidence for remaining integration, and says current findings do not settle whether consciousness is unified or split (de Haan et al., 2020).
Nagel asks how to describe the patient’s experience. Is there one subject with divided access, two streams of experience, partial unity, or something that does not fit these options?
What do experimental dissociations and everyday behavioral integration tell us about the number and unity of conscious streams in a split-brain patient?
Nagel’s 1971 argument weighs the strong integration visible in ordinary behavior against the dissociations elicited in controlled experiments. He argues that these bodies of evidence do not support a straightforward whole-number count of minds, and that the case exposes limits in ordinary ideas of a single subject of experience. This is Nagel’s philosophical conclusion, not a settled neuroscientific result. Read the paper alongside the current SEP discussion of split-brain models.
The case has shaped debates about phenomenal unity and subjecthood. The experimental findings alone do not decide questions about a person’s identity over time. Variants such as reconnecting a divided brain are philosophical extensions, not clinical evidence.
The main proposals include two-stream views, single-stream views (including a switch model), partial-stream views, and hybrids that treat laboratory and everyday settings differently. The SEP overview explains that these models make different claims about the structure of experience; choosing a model does not by itself settle whether the patient remains one subject.
Nagel’s own 1971 argument is distinct: he contends that ordinary concepts of mind and person do not yield a determinate one-or-two answer for these cases. Philosophers and researchers continue to disagree about how the evidence should be interpreted.
The case does not show that ordinary people have two minds, or that each hemisphere is a separate person. The classic findings come from controlled tests, and patients differ in the extent and type of disconnection. In a 2017 study of two patients with full callosotomy, Pinto and colleagues found that both could detect and locate stimuli presented across the visual field regardless of response mode. In a separate cross-hemifield comparison task with Patient DDC, performance was not significantly above chance, consistent with a failure to integrate the two visual inputs. The authors argued that their broader results support undivided consciousness; this interpretation is contested, not a settled refutation of earlier dissociation findings (Pinto et al., 2017). The broader empirical and philosophical disagreement is reviewed by de Haan et al. (2020).
One can imagine reconnecting a divided brain or performing the operation on healthy people. These are speculative thought experiments, not established procedures or findings. They should be kept separate from the clinical cases.
Relevant passages of Nagel’s full paper, including its argument about conflicting behavioral and experimental evidence and its conclusion about ordinary concepts of mind and person, were consulted. Relevant passages of the 1962 and 1968 primary reports, the SEP discussion of split-brain models, and the open-access 2020 review were also consulted. The publisher page for Gazzaniga, Bogen, and Sperry (1962) provided the full article text. Pinto et al. (2017), “Patients and methods—Subjects/Procedure,” “Results” (Experiments 1–2), and “Discussion” were consulted for the study’s detection/localization and comparison results; its unified-consciousness conclusion is reported as the authors’ interpretation, not as a settled result. Everyday behavior is not characterized here as fully normal, and philosophical implications for identity over time are not presented as direct empirical findings.
Research check: 2026-11-01. This is an educational explanation, not expert review or a substitute for the primary texts. Additional comparisons are teaching prompts, not established historical variations.