How can you have objective evidence for pure rationality?

Pure rationality is a mathematical ideal, not a biological fact. No study has ever found a brain that reasons without emotion, bias, or bodily influence.

The retrieved evidence shows that human decision-making is always a mix of emotion and logic. A 2024 study of 79 military nursing cadets found that emotionally urgent decisions predicted higher stress during clinical practice, while rational urgent decisions did not [1]. The two modes are distinct and both present. A 2019 experiment with 30 participants in a multiplayer game found that public popularity signals did not reliably predict choices, and risk-taking increased when the objects were faces instead of boxes or cars [2]. People systematically deviated from pure rationality.

Even training to reduce bias does not produce pure rationality. A 2024 study gave participants a debiasing intervention and measured accuracy on three classic reasoning tasks: bat-and-ball, base-rate neglect, and conjunction fallacy. The training group improved from 84.3% to 93.5% overall accuracy [3]. That is better, but still not perfect. The control group also improved, from 85.9% to 87.7%, just from taking the test twice [3]. The ceiling is not 100%.

The evidence also shows that rationality can be impaired by brain disease. A 2013 study found that people with obsessive-compulsive disorder made rational decisions on only 87.3% of trials, compared to 98.4% for healthy controls [4]. Even healthy brains have a measurable failure rate. A 2025 study of ChatGPT found it showed only partial rationality: it aligned with the independence axiom but struggled with transitivity and Savage's Sure Thing principle [5]. A language model trained on the entire internet cannot sustain pure rationality either.

My call: pure rationality is an abstract standard used to measure how far real minds fall short. No human or machine has ever met it. Confidence: high.