An (incomplete) overview of the factors relevant to reaching sufficient constraining power in a weak- lensing survey and their relationship to the top-level weak-lensing requirements in the Science Requirements Document (Euclid Collaboration 2015). Once sufficient statistical precision is reached, attention has to be paid to astrophysical biases arising from the nature and disposition of the contents of the Universe, those arising from the observational strategy and the observations themselves, and those from the cosmological modelling. The final accuracy of the experiment is set by the residuals after treatment of all of these effects.