Source: The inclusion of the Other. Studies in Political Theory. Jürgen Habermas. MIT Press, 1998, parts VIII and IX of Chapter 1 only, reproduced here;
Transcribed by: Andy Blunden.
"It is no accident that the categorical imperative is directed to the second person singular and that it creates the impression that each individual could undertake the required test of norms for himself in foro interno. But in fact the reflexive application of the universalisation test calls for a form of deliberation in which each participant is compelled to adopt the perspective of all others in order to examine whether a norm could be willed by all from the perspective of each person. This is the situation of a rational discourse oriented to reaching understanding in which all those concerned participate. This idea of a discursively produced understanding also imposes a greater burden of justification on the isolated judging subject than would a monologically applied universalisation test."
Read the parts in full here