We are currently in a period entirely devoted to Janus. I think it may be useful to make some reflection about the Arch as peculiar symbolic expression of the idea of "passage".
Janus represents, among other, the sacralization of the "passage" in a terms of space, time and initiation (the vessel). Related to Time, Janus is the sacralization of the moments of passage or "phase change" during natural cycles, in the human life, etc. Related to Space, the doors, the gates and all the sites materializing a passage have a sacred energy embodied in Janus. As for the time passages, the spatial passages represent a phase modification, a fracture and a discontinuity: the door, for example, divides and put in contact two spaces having different identities. In particular the door of the Temple (both architectural and natural) is a break-point between an "in" and an "out".
Passing through a door is a changing of status: this is particular relevant for a sacred place like a Temple but it is the same also for the house door.
I prefer not to discuss here about the meaning of the passage in initiation terms because it is a too long and complex issue to be properly analyzed here.
Therefore "passing through the door" is always a "dramatic" moment (in sacred sense): above all the passage through the "narrow gate" (observe the etruscan tombs or the ctonic sactuaries - palingenesis) and the bridges (structures unifying what Nature divided).
The Arch can be classified among those architectural structures with sacred and symbolic functions having a specific and peculiar meaning. Like other "passages", the arch presupposes a phase change, a continuity interruption, an "after-before" condition, a fracture in space and time. Unlike other types of passages, the Arch depicts a "purification passage", a purification from the extreme impurity. It is thus an essential tool to restore a purity status which has been somehow altered, a condition of order which has been violated, a restoring of Cosmos from Chaos.
The purifyng function of the Arch is extremely complex according to a symbolic point of view and it is very difficult to discuss briefly this topic for the presence of many related issues.
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Therefore, a triumph was not a military parade but a sacred rite whose main goal was the restoring of the Order the war and the bloodshed had altered.
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Similar considerations (the purification of the foreigners and the purification of "what is outside" the consacrated city space) could be made for the town gates.
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Passing through an arch hence is not a "neutral" operation: reflecting about this function means maintaining a status of active continuos awareness in order to achieve a constant sacred and oriented vision of our life.