1. Extension and Intension.- 1.1 The Basic Doctrine.- 1.2 A Set-theoretic Formulation.- 1.3 Extension and Intension in Formalized Theories.- 1.4 Intension as Comprehension.- 1.5 Calculi of Extensions and Intensions.- 1.6 Extension and Intension of Theories.- 1.7 Intension as Connotation: Core Intension.- 1.8 Vagueness.- 1.9 Intensional Autonomy.- 2. Meaning.- 2.1 Correspondence and Coherence Views.- 2.2 Meaning as Intension/Extension.- 2.3 Meaning of Constructs in Mathematical Theories.- 2.4 Meaning in Formal Theories.- 2.5 C. I. Lewis on Meaning.- 2.6 Truth in Theory and Truth in Practice.- 2.7 Nonexistent Possibles.- 3. Existence.- 3.1 The Thesis that Existence is Consistency.- 3.2 Empiricist Notions of Existence.- 3.3 Objectivity and Evidence.- 3.4 A Seasoned Constructivism: Piaget's Genetic Epistemology.- 3.5 Heuristics and Mathematical Existence.- 3.6 Style.- 3.7 Sets and the Semantics of Mathematics.- 3.8 Categories and the De-ontologization of Mathematics.- 4. Reduction.- 4.1 Reduction in Mathematics.- 4.2 Meaning-preserving Correspondences.- 4.3 Explanation v. Reduction.- 4.4 Ontological Commitment.- 4.5 Ontological Reduction.- Index of Names.- Partial List of Symbols.