Symbols - What does heaven look like
In most recent symbolism the ass, along with the donkey, symbolised patience, fortitude, humility and courage, as well as an ability to withstand a lot of ill-treatment with little complaint. In a sense the ass is the personification [or animalisation!] of the expression 'turn the other cheek'.
But in the past, the ass was the animal of Saturn. Saturn, being the Planet symbolically closest to the Sun was regared as a rival of the Sun. Saturn's qualities include a considerable amount of fairly hard hearted intelligence and as such the rivalry between the two was mythology often quite intense.
Saturn was often an Intelligence revered by shamanic societies on the basis that the Ultimate Intelligence was unknowable ad unreachable, but Saturn was accessible, as such Saturn tended to be the god associated with Jesters, magicians and jokers. And it is from this association that the ass gets its 'silly ass' symbolism.
There appears also to have been a concerted effort on the part of the religions that wished to stamp out shamanism and replace it with moral based frameworks to deliberately make the ass [and the goat] into symbols of ridicule - see also Satan.
J E Cirlot - A Dictionary of symbols
The ass appears as an attribute of Saturn, in his capacity as the 'second sun'. It is always on heat ad hated by Isis. The significance of the mock crucifix with an ass's head from the Palantine must be related to the equation of Yahve with Saturn...
In connection with the jester symbol, the ass's head is frequently found in medieval emblems, sometimes with a wheel or a solar symbol between the ass's ears.... In dreams, the ass, especially when it appears invested with a solemn and riual aspect, is usually a messenger of death or appears in connection with a death
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