 Frege's definitions of numbers, lines, and shapes in terms of extensions can be formulated as follows: (i) the number of Fs is the extension of the concept equinumerous with F (that is, it is the set of all concepts that have exactly as many objects falling under them as does F); and (ii) the direction of line a is the extension of the concept parallel to a; and (iii) the shape of figure a is the extension of the concept geometrically similar to a. A similar approach can be used to define other kinds of logical objects. For instance, the truth value of the proposition p can be identified with the extension of the concept equivalent to p (i.e., the concept true if and only if p is true).