  We introduce a method to obtain one-dimensional collective variables for
studying rarely occurring transitions between two metastable states separated
by a high free energy barrier. No previous information, not even approximated,
on the path followed during the transition is needed. The only requirement is
to know the fluctuations of the system while in the two metastable states. With
this information in hand we build the collective variable using a modified
version of Fisher's linear discriminant analysis. The usefulness of this
approach is tested on the metadynamics simulation of two representative
systems. The first is the freezing of silver iodide into the superionic
$\alpha$-phase, the second is the study of a classical Diels Alder reaction.
The collective variable works very well in these two diverse cases.
