Why is intellectual work not considered servile work?

Why is intellectual work not considered servile work prohibited by the 3rd Commandment on Sundays and feast days? Doesn't the soul need rest just as much as the body those days? St. Thomas, defending the necessity of recreation , said "weariness of the soul must needs be remedied by resting the soul"; intellectual work can be physically exhausting ; and the *Primitive Constitutions of the Order of Friars Preachers* (c. 1228) prohibits writing manuscripts on Sundays and feast days. Why has servile work come to mean solely heavy manual (bodily) labor?