Compared with other fields of human endeavour, mathematics is usually
regarded as a subject of great precision in which concepts can be defined
accurately to provide a firm foundation for the mathematical theory. The
psychological realities are somewhat different. Many concepts we meet in
mathematics have been encountered in some form or other before they are
formally defined and a complex cognitive structure exists in the mind of every
individual, yielding a variety of personal mental images when a concept is
evoked. In this paper we formulate a number of general ideas intended to be
helpful in analysing these phenomena and apply them to the specific concepts of
continuity and limits.