2-4-6 problem n. A problem of concept formation in which people are given the ordered triple of numbers (2, 4, 6) and are invited to try to generate further examples of triples conforming to an unspecified rule that the example obeys, trying to home in on the correct rule on the basis of simple right/wrong feedback after every guess. The actual rule is any ascending sequence , but the example invites people to form more specific hypotheses, such as ascending even numbers or numbers ascending by equal ...