Relational cate-gories are categories for which the basis for membership is participation in a commonrelational structure; thus, they differ from the more studied entity categories, such as tulipand spoon, whose members share many intrinsic properties.

Our main focus is on relational abstractions, includingprinciples, rules, and schemas, as well as abstract relational categories.

For example, causal system is more abstract than posi-tive feedback system, which in turn is more abstract than the specific positive feedbacksystem by which the melting of polar ice causes lower reflectance of the sun’s heat, lead-ing in turn to more rapid melting.

Wetake the process of abstraction to be one of decreasing the specificity (and therebyincreasing the scope) of a concept.

Many such abstractions are expressed as rela-tional categories—categories like evidence, counterfactual, and proportion, and on a moremundane level, bargain, ally, and rescue.

Theassertions that make up abstract knowledge are variously referred to as schemas, rules,abstractions, principles, or overhypotheses

Abstract structured knowledge is a key feature of higher order cognition (Gentner &Medina, 1998; Hummel, 2011; Markman, 1999; Tenenbaum & Griffiths, 2001).