Without conversation and challenge, philosophy very quickly lapses into the dead dogma that Mill feared.
Dogma: a set of principles, ideas, or beliefs that are said to be incontrovertibly true. Our principles, ideas, and beliefs should be challenged. When we are not questioning or developing them we are likely to fall into a rut of living how we always do and believing what we always have. We will stop expanding our thoughts and ideas if we don’t question our current ones.