This book seeks the unity in this diversity by asking what properties a structure should have to qualify it to be a logic. It does not provide a complete atlas of the logic manifold but prepounds a principle which, though it makes neither the crooked straight nor the rough places plane, facilitates the drawing of local charts of the fortuitous crookedness of real logical highways and byways.

The multiplicity of logics; Classical logic; Abstract logics; Logical operations; Order and lattices; Constructing logics; Quasi-Boolean algebras and empirical continuity; Three-valued logic; Relevance; The calculus of logics: effective logic; Modal logics; Appendix; References; Index.