function of model-independent observables, and thus a model-independent measurement of $\eta$, valid under our assumptions (*a*)-(*d*), but *not* requiring (*e*). The form of the last term is determined by the QS limit of the Horndeski Lagrangian. Given the above, we can exclude all dark energy models described by a single scalar field in the QS limit by showing that the anisotropic stress measured from the observation data does not follow the particular scale dependence mandated by Eq. . Equation  must be valid in fact at all times and scales where the quasi-static limit is valid. At any given epoch $z^{*}$, this equation involves the three unknowns $h_{2},h_{4,}h_{5}$ all evaluated at $z^{*}$. If at this epoch we observe $E$ and $P_{2},P_{3}$ at more than three $k$-modes, we can form an overconstrained system. If for any $z^{*}$ this system has no solution then the observations are inconsistent with the quasi-static limit of HL. Equivalently, from Eq.  one can obtain a consistency relation that depends only on observable quantities. Defining $g(z,k)\equiv\frac{(REa^{2})'}{LEa^{2}}$, one has in fact $$2g^{(1)}g^{(3)}-3(g^{(2)})^{2}=0\,,\label{eq:cons}$$ where $g^{(n)}$ is the $n$-th derivative of $g$ with respect to $k^{2}$. If this condition fails at any one redshift, the DE is not described by the 