Introduction.- Part I Co(X) and B(H): Classical physics on a finite phase space.- Quantum mechanics on a finite-dimensional Hilbert space.- Classical physics on a general phase space.- Quantum physics on a general Hilbert space.- Symmetry in quantum mechanics.- Part II Between Co(X) and B(H): Classical models of quantum mechanics.- Small hbar.- large N .- Symmetry in algebraic quantum theory.- Spontaneous Symmetry Breaking.- The Measurement Problem.- Topos theory and quantum logic.- Appendix Finite-dimensional Hilbert spaces.- Appendix Basic functional analysis.- Appendix Operator algebras.- Appendix Lattices and logic.- Appendix Category theory and topos theory.- References.