How did the US Supreme Court feel about the idea of “separate but equal” in the case Plessy v. Ferguson?

Why does c. wright mills identify critical thinking as one of the most important criteria for the development of intellectual craftsmanship? why is constantly being aware of the biased nature of your reactions and interactions (what c. wright mills calls self-reflexive) key to the development of that critical thinking? finally, why is keeping a file important, how can it lean to intellectual curiosity which can foster critical thinking and to develop a keen analytical mind?