The fundamental meaning of the 4th Key is Reason. The key is called the Emperor, and the Emperor is one who oversees and controls. This is the basic function of reason, for by it we supervise and control our daily activities. Vision is also attributed to this card and has a close connection with the faculty of reason, when we consider reason as "insight" or mental vision. Physical sight is also meant here, and the orange background is symbolic of the Sun, source of the light by means of which we are able to see.
The mountain represents the fiery activity of Mars and Aries, which are attributes of this Key. Aries is also symbolized by the rams' heads on shoulder and throne, and Mars by the steel armor with which the Emperor is clad. The twelve points that ornament the helmet are also astrological.
The globe is a conventional symbol of dominion. This globe and cross, like the circle and Tau of the sceptre, symbolize the masculine and feminine potencies of the radiant mental energy, or Life-Force.
The purple flaps over his armor hint at his rulership, since purple is the color of royalty.