The Age of the World and Creation
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'Both the druids and others assert that both the soul and the world are indestructible, but that sometimes fire and sometimes water have overwhelmed them.' - Strabo, Geography 4.4.4
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With this quote we an infer quite a few things about the world. For one it seems to be thought that the world, as in the universe, could not be destroyed and likely that it had no clear origin or destruction. Not that the Celts thought was uncreated, rather that the universe that the world and everything know in it couldn't properly be ended and also was not created.
The Second part of the quote points towards how the Celts thought time flowed. That Fire and Water would destroy the world, but seemingly not the universe and that cannot be properly destroyed. Therefore it seems that the world is subject to multiple instances of creation and destruction, a similar cycle of creation and destruction is found in the Aztec Religion and Hinduism.
It is likely that there was some lore on what came before our world however this has decisively been lost. In the end however we still get this model for how the universe functions and will continue to function long after our world ends.