  A classical result due to Segre states that on a real cubic surface in
${\mathbb P}^3_\R$ there exists two kinds of real lines: elliptic and
hyperbolic lines. These two kinds of real lines are defined in an intrinsic
way, i.e., their definition does not depend on any choices of orientation data.
Segre's classification of smooth real cubic surfaces also shows that any such
surface contains at least 3 real lines. Starting from these remarks and
inspired by the classical problem mentioned above, our article has the
following goals:
  - We explain a general principle which leads to lower bounds in real
algebraic geometry,
  - We explain the reason for the appearance of intrinsic signs in the
classical problem treated by Segre, showing that the same phenomenon occurs in
a large class of enumerative problems in real algebraic geometry.
  - We illustrate these principles in the enumerative problem for real lines in
real hypersurfaces of degree $2m-3$ in ${\mathbb P}^m_\R$.
