  Zero-point energy is generally known to be unphysical. Casimir effect,
however, is often presented as a counterexample, giving rise to a conceptual
confusion. To resolve the confusion we study foundational aspects of Casimir
effect at a qualitative level, but also at a quantitative level within a simple
toy model with only 3 degrees of freedom. In particular, we point out that
Casimir vacuum is not a state without photons, and not a ground state for a
Hamiltonian that can describe Casimir force. Instead, Casimir vacuum can be
related to the photon vacuum by a non-trivial Bogoliubov transformation, and it
is a ground state only for an effective Hamiltonian describing Casimir plates
at a fixed distance. At the fundamental microscopic level, Casimir force is
best viewed as a manifestation of van der Waals forces.
