The 'One Drop Rule' --- the American laws that if you had ever had any non-white fore bearers in your distant past, one drop of black* blood in you, then you were not 100% pure white and hence had to be completely black instead, are now seen by most as a great moral evil.
They were also very good examples of the dangers of relying upon logical thinking that avoids checking in on the real world.
Those laws were all based upon following up the consequences leading from their primary assumption, the seemingly unambiguous definition of the word "pure".
(*In Whites Only Australia and in Latin America, interestingly, the tendency was if the person had one drop of white blood then they were white --- equally real world absurd but in the other direction.)
The problem was that definitions like clean and pure are those Platonic words of absolute absolute - things can only be 100% pure or 100% clean or they are no longer pure or clean, by definition*.
(*Pure : not mixed with any other substance.)
And because avoiding having a few stray atoms of other matter cling to even the purest of scientific samples is impossible, nothing in the real world is really pure or really clean.
At best, it can only be relatively clean and relatively pure --- or equally convincingly, it can only be described as relatively dirty and relatively impure.
All matter, including us humans, is therefore really arranged on a long continuum made up of varying shades of grey --- not 100% clean white and 100% dirty black.
Unmodified words like clean and pure therefore are best understood as the external manifestations of an internal mental affliction.
They are also examples of a long history of thinkers seeking to impose unreal Platonic and universal definitions upon our shared real world, all in order to salve an internal and personal mental condition suffered by only themselves and their fellow afflicted.
Prozac relief by dictionary, as it were.
Thinkers who tend to see the world in broadly Platonic ways all suffer from a common mental condition shared by many others, that of having a marked intolerance for ambiguity.
We recognize this affliction also among people who are politically conservative, or rigid in personality, or among a common subset of scientists, philosophers and mathematical-oriented types.
And to these poor people, the fact that words like clean and pure have no real world equivalent is beyond cognitive acceptance.
And to some of these afflicted folk in our recent past, the mere fact that anyone competent in both logic and with a knowledge of history could demonstrate that if we humans all came 'out of Africa', then we must all have at least 'one drop of Negro blood' in our distant past, was totally without consequence.
They badly need mental relief and this law used to provide it --- for everyone was now back to being either fully black or fully white, neatly reflecting the way these afflicted folk viewed the entire world.
These 'one drop' laws has been abandoned (in theory) but interestingly the FBI (as you might have guessed !) still does not recognize mixed race as a self definition - one is either white, black or yellow etc.
The FBI is now in the 21st century and recognizes and accepts many things that it didn't in the past --- but along with its fellow afflicted sufferers in conservative parties and in branches of academia, accepting ambiguity is not one of them....