  A phenomenological criterion for the superfluid transition is proposed, which
is similar to the Lindemann criterion for the crystal melting. Then we derive a
new formula for the critical temperature, relating $T_{\lambda}$ to the mean
kinetic energy per particle above the transition. The suppression of the
critical temperature in a sufficiently dense liquid is described as a result of
the quantum decoherence phenomenon. The theory can account for the observed
dependence of $T_{\lambda}$ on density in liquid helium and results in an
estimate $T_{\lambda} \sim 1.1$ K for molecular hydrogen.
