  Liouville's 1853 paper, in which he derived in closed form the general local
solution of equation $u_{z\bar z}=\exp(u)$, is one of the few papers from the
19th century that 21st century mathematicians routinely quote as motivation for
their work. We try and understand the reasons for the enduring importance of
this paper. Our conclusions are the following: 1. Mathematics is not
cumulative, but lost material may be recovered through examination of ancient
works. 2. An apparent paradox is that the degree of dissemination of ideas and
concepts among mathematicians is not determined only by geographic or social
affiliation (countries, cities, scientific schools, etc.). On the contrary, the
existence of common scientific goals makes it possible to transfer and use
these concepts. 3. Older papers, because they are not influenced by modern
classifications, can help generate ideas that do not arise naturally in a
modern framework. Thus, a historical perspective can help bring about new
mathematics and indeed, is a necessary dimension of mathematical research.
