Potamon fluviatile as well as on the island of Sicily. Although it used to be found as far north as Lake Garda, "P. fluviatile" no longer occurs north of the River Po. In 1997 a population of "P. fluviatile" was discovered under the ruins of Trajan's Forum in the heart of Rome, living in canals built by the Etruscans which connect to the "Cloaca Maxima". Based on a genetic analysis, which demonstrated that these crabs were similar to those in Greece, researchers believe that they had been brought by the Greeks before the founding of the city, some 3000 years ago. The
Problem of why there is anything at all that the question cannot have a causal explanation (as any cause must itself have a cause) or a contingent explanation (as the factors giving the contingency must pre-exist), and that if there is an answer it must be something that exists necessarily (i.e. something that just exists, rather than is caused). Gottfried Wilhelm Leibniz wrote: ""Why is there something rather than nothing? The sufficient reason [...] is found in a substance which [...] is a necessary being bearing the reason for its existence within itself."" Philosopher of physics Dean Rickles has argued that numbers and mathematics (or their underlying laws)