ABSTRACT
I must confess that I feel some embarrassment in presenting these concepts, because they are mostly taught nowadays in high-school. I took this decision, however, partly in order to avoid any ambiguity in the use of terms, which frequently have different meanings according to the school of thought; and partly because the definitions of the earlier part of this chapter, which are the most widely known, are necessary for those of the latter part, with which, so far as I am aware, people are not yet as familiar.
