We study logical reduction (factorization) of relations into relations of lower arity by Boolean or relative products that come from applying conjunctions and existential quantifiers to predicates, i.e. by primitive positive formulas of predicate calculus. Our algebraic framework unifies natural joins and data dependencies of database theory and relational algebra of clone theory with the bond algebra of C.S. Peirce. We also offer new constructions of reductions, systematically study irreducible relations and reductions to them and introduce a new characteristic of (...) relations, ternarity, that measures their ‘complexity of relating’ and allows to refine reduction results. In particular, we refine Peirce’s controversial reduction thesis, and show that reducibility behaviour is dramatically different on finite and infinite domains. (shrink)

We argue that traditional formulations of the reduction thesis that tie it to privileged relational operations do not suffice for Peirce’s justification of the categories and invite the charge of gerrymandering to make it come out as true. We then develop a more robust invariant formulation of the thesis, one that is immune to that charge, by explicating the use of triads in any relational operations. The explication also allows us to track how Thirdness enters the structure of higher order (...) relations and even to propose a numerical measure of it. Our analysis reveals new conceptual phenomena when negation or disjunction are used to compound relations. (shrink)