The following passage comes from the majority opinion of Plessy v. Ferguson (1896):
The argument ... assumes that social prejudice may be overcome by legislation, and that equal rights cannot be secured except by an enforced commingling of the two races... If the civil and political rights of both races be equal, one cannot be inferior to the other civilly or politically. If one race be inferior to the other socially, the Constitution of the United States cannot put them upon the same plane.
I learned that this ruling legalized segregation and normalize the idea of "separate but equal."
What I have trouble understanding is the last sentence highlighted in bold.
What does it mean, in context, for the US Constitution not being able to put them (two races) upon the same plane. Does it mean in the eyes of the Constitution, the two races are not seen as equal (that is, not on the same plane)? But then, the idea that was popularized was "separate but equal" so now, I'm confused.