What do we call a rule? Consider this example:
33).   B moves about according to rules which A gives him. B is supplied with the following table:
a
b
c
d





A gives an order made up of the letters in the table, say: “a a c a d d d”. B looks up the arrow corresponding to each letter of the order and moves accordingly; in our example thus:















The table 33) we should call a rule (or else “the expression of a rule”. Why I give these synonymous expressions will appear later.) We shan't be inclined to call the sentence “a a c a d d d” itself a rule. It is of course the description of the way B has to take. On the other hand, such a description would under certain circumstances be called a rule, e.g., in the following case:
30.
34).   B is to draw various ornamental linear designs. Each design is a repetition of one element which A gives him. Thus if A gives the order “c a d a”, B draws a line thus: