  It is shown that a k=0, A_{2u} distortion of the terbium tetrahedral network
in Tb2Ti2O7 accounts for the apparent isolation of single tetrahedra as seen in
neutron scattering studies. Single tetrahedron collective spin states, rather
than individual spins, account for the main features of the spin liquid state,
namely, fluctuating local moments and the absence of long range order. Singlet
and doublet collective spin ground states are considered. An effective
interaction between tetrahedra on the fcc lattice is derived and found to be
weak and anisotropic.
