Laws of form

Blurb

Laws of Form is a book by G. Spencer-Brown, published in 1969, that straddles the boundary between mathematics and philosophy. LoF describes three distinct logical systems:
The primary arithmetic, whose models include Boolean arithmetic;
The primary algebra, whose models include the two-element Boolean algebra, Boolean logic, and the classical propositional calculus;
Equations of the second degree, whose interpretations include finite automata and Alonzo Church's Restricted Recursive Arithmetic.
Boundary algebra is Dr Philip Meguire's term for the union of the primary algebra and the primary arithmetic. "Laws of Form" sometimes loosely refers to the pa as well as to LoF.