  In this paper, the foundations of classical phenomenological thermodynamics
are being thoroughly revisited. A new rigorous basis for thermodynamics is laid
out in the main text and presented in full detail in the appendix. All relevant
concepts, such as work, heat, internal energy, heat reservoirs, reversibility,
absolute temperature and entropy, are introduced on an abstract level and
connected through traditional results, such as Carnot's Theorem, Clausius'
Theorem and the Entropy Theorem. The paper offers insights into the basic
assumptions one has to make in order to formally introduce a phenomenological
thermodynamic theory. This contribution is of particular importance when
applying phenomenological thermodynamics to systems, such as black holes, where
the microscopic physics is not yet fully understood. Altogether, this work can
serve as a basis for a complete and rigorous introduction to thermodynamics in
an undergraduate course which follows the traditional lines as closely as
possible.
