Reasoning is the core activity for a retrospective. But it is important for a group to have a common ground on what they are reasoning about. The group has to first look at facts and perceptions.
All argument and reasoning must be based upon certain perceptions. Without these, there cannot be any argument. Reasoning is the method of comparison between certain facts which the group has already perceived. If these perceived facts are not there already, there cannot be any reasoning.
When planning a retrospective, choose an activity to help the team uncover facts. Facts help the group understand perceptions, and upon that the group will be ready to engage into reasoning.