The Spock rule is a metarule used in Zendo. A Master's secret rule is allowable in Zendo if (and only if) the rule could be applied by the fictional character Spock. Under this scenario, a koan is "beamed up" to Spock, who knows the rule and can always correctly mark the koan and beam it back to the players.
The Spock rule therefore allows only objectively observable attributes of a koan to be relevant. Since Spock sees nothing except the pyramids and the way they stand together on a surface, the Spock rule is a convenient way to explain to beginners about the possible scope of Zendo rules.
These are examples of would-be Zendo rules outlawed by the Spock rule:
- A koan has the Buddha nature if...
- somebody touched it with their left hand
- any pyramid points East (Spock can't see which way is East)
- any pyramid is the same as in another koan
- it's built on a crack in the table
- the marking stone is between the koan and the master
These are most of the characteristics which may be used (singly, or in combination) in compliance with the Spock rule: