Designed as the left-hand wing of the triptych, The Age of Reason depicts the cold and ruthless aspect of human nature, in contrast to the right-hand wing, 'The Age of Love'.

According to his analysis of the three human attributes in his sketchbook, reason is materialistic, intolerant and competitive; it believes in its superiority and it desires to rule, hence it seeks war.

In this study, Mucha captured a nightmarish vision of a world ruled by reason, symbolised by a church, giant wheels, machines and factory chimneys; the world is packed with angry crowds while a handful of authorities and dictators control the masses by reason.