ABSTRACT
Two parallellines constitute aself-congruentfigure. Wecannot,however, immediately assert that a copy shifted in the direction of their length remains in the position of the originals. For we have first to show that it is possible to construct two straight lines, simultaneously or in succession, which are self-congruent in this way; and it is not (at this stage of our analysis) self-evident that the possibility is implied by our operational definition of a single straight line.
