Of course, al-Farabi realized that such a society was rare and required a very specific set of historical circumstances to be realized, which means very few societies could ever attain this goal. He divided those "vicious" societies, which have fallen short of the ideal "virtuous" society, into three categories: ignorant, wicked and errant. Ignorant societies have, for whatever reason, failed to comprehend the purpose of human existence, and have supplanted the pursuit of happiness for another (inferior) goal, whether this be wealth, sensual gratification or power. Al-Farabi mentions "weeds" in the virtuous society: those people who try to undermine its progress towards the true human end. The best known Arabic source for al-Farabi's political philosophy is his work titled, Ara Ahl al-Madina al-fadila.