The September commission
Berlin, 1904. In a paved courtyard in the north of the city, between tall apartment buildings, a horse answers visitors’ questions by tapping his hoof on the ground. Hans belongs to Mr. von Osten, a former mathematics teacher who has spent years teaching him. In Oskar Pfungst’s later account, demonstrations take place around noon, with no admission charge. Visitors can approach the owner and his horse, rewarded with bread or carrots. More than a performance is at stake: if Hans really solves the problems put to him, a horse could possess a power of reasoning that many reserve for humans.
On September 12, thirteen signatories issue their findings. They include Paul Busch, a circus director, and Carl Stumpf, director of Berlin’s Psychological Institute. They have looked for a trick: a movement or sign deliberately used to supply the answer. Their report, reproduced in Pfungst’s book, rules out that explanation under the conditions of their inquiry:
After a careful investigation they are unanimously agreed that such signs are out of the question under the conditions which were maintained during this investigation.
The finding favours Hans, but it does not certify that he calculates. The signatories also say they have excluded familiar kinds of involuntary signs and report successes when the questioner does not know the solution. This is an observation to test, not an explained mechanism. Oskar Pfungst, who conducts the psychological experiments, will revisit the difficulty through procedures with and without knowledge of the answer. In the Berlin courtyard, the hoof gives a number; what tells it to stop has yet to be established.
Clipped adaptation with T01 in original, scan-verified English. September commission report reproduced by Oskar Pfungst, printed p. 253; PDF p. 263; exact Cornell-BHL JP2 leaf 0262. The conclusion is limited to the maintained conditions. Narrative context: the same Cornell-BHL volume, pp. 13, 18, 34–35, 42–45, 57–62, 102–103, 133–139 and 253–265; see the narrative review ledger.