According to d’Alembert’s paradox, in a vortex-free and frictionless state—such as immediately before an aeroplane takes off—no aerodynamic lift is possible. It will now be demonstrated that the formation of vortices is only possible through friction. Since the corresponding processes usually only occur in a thin boundary layer, the vortices formed can be limited to the boundary of the wing profile using the concept of circulation. This leaves the surrounding space almost vortex-free and allows the use of potential theoretical methods. After examining the conditions for flow separation, the creation of lift in Wagner’s model is explained.