  We derive a lower limit to the amount of absorptive loss present in passive
linear optical devices such as a beam splitter. We choose a particularly simple
beam splitter geometry, a single planar slab surrounded by vacuum, which
already reveals the important features of the theory. It is shown that, using
general causality requirements and statistical arguments, the lower bound
depends on the frequency of the incident light and the transverse resonance
frequency of a suitably chosen single-resonance model only. For symmetric beam
splitters and reasonable assumptions on the resonance frequency $\omega_T$, the
lower absorption bound is $p_{\min}\approx 10^{-6}(\omega/\omega_T)^4$.
