Incluye referencias bibliográficas (páginas 240-242) e índice. CONTENIDO: Introductión: the roots of my theory -- Preliminary questions -- How do we meet a structure? -- How do we recognize a structure? -- Rigid and feeble structures -- What is a structure? -- Properties os a structure -- The media in which structure acts -- Levels of thinking -- The consequences and a more exact analysis of the theory of levels of thinking -- A psychological approach to levels of thinking -- Intuition -- Reality, individual, and language -- Structures at different levels -- Consequences of the languages at different levels -- The development of number -- Levels of structure in argument -- The intuitive foundations of mathematics -- Direct reactions to visual structures -- Direct interactions between reality and common human knowledge -- Switching over from one structure to another -- The relations among the media of structure -- Is it possible to test insight? -- A phenomenal introduction to geometry -- The significance of intention in the learning process -- The problem of motivation in education -- System separation and transfer -- The relation between theory and problems in arithmetic and algebra -- Proportion and fraction -- Objectivity -- A step in the direction of philosophy.