The line [math]c[/math] is non-selfconjugate, the projectivity [math]X\bar \wedge Y[/math], where [math]Y=c\cdot x[/math] transforms any non-selfconjugate point [math]B[/math] into another point[math]A =b\cdot c[/math] whose polar is [math]BC[/math]. This same projectivity transforms A into B. Since it interchanges [math]A[/math] and [math]B[/math], it must be an involution.