Laws of Form
Laws of Form was listed in the Whole Earth Catalog in 1969 and quickly became a cult classic.  The calculus of indications and the primary algebra may be regarded as a way to think about a fundamental activity of the mind, namely, the ability to distinguish or to draw distinctions.  The book argues that this ability is the foundation of human cognition and consciousness.  According to Spencer-Brown, the primary arithemetic and primary algebra reveal new connections among logic, mathematics, the philosophy of language, and the philosophy of mind.