  In this article we prove that the Klein-Gordon equation in the de Sitter
spacetime obeys the Huygens' principle only if the physical mass $m$ of the
scalar field and the dimension $n\geq 2$ of the spatial variable are tied by
the equation $m^2=(n^2-1)/4 $. Moreover, we define the incomplete Huygens'
principle, which is the Huygens' principle restricted to the vanishing second
initial datum, and then reveal that the massless scalar field in the de Sitter
spacetime obeys the incomplete Huygens' principle and does not obey the
Huygens' principle, for the dimensions $n=1,3$, only. Thus, in the de Sitter
spacetime the existence of two different scalar fields (in fact, with m=0 and
$m^2=(n^2-1)/4 $), which obey incomplete Huygens' principle, is equivalent to
the condition $n=3$ (in fact, the spatial dimension of the physical world). For
$n=3$ these two values of the mass are the endpoints of the so-called in
quantum field theory the Higuchi bound. The value $m^2=(n^2-1)/4 $ of the
physical mass allows us also to obtain complete asymptotic expansion of the
solution for the large time. Keywords: Huygens' Principle; Klein-Gordon
Equation; de Sitter spacetime; Higuchi Bound
