Shades of Grey
These two very different subjects have at their heart the same problem - what should be a simple black and white concept, isn't. Is the person male or female? Is the person alive or dead?
So you'd think this is lunacy - have them drop their drawers and take a peek. How can you not know if they're male or female? Well, so you do this with a girl of 8 and you say, she is definitely female. Then she hits puberty, and all of a sudden, she has testes which descend into her labia, her clitoris elongates into a short penis, and voom - she became a he. Without going into details, it seems that no approach you take, physical examinations, hormone testing, genetic testing, is going to give you a clear cut answer.
And this is what freaks me out. Humans make sense of the world hierarchically. We start with things we're sure of and build on them. However, the more I look into the things that we're supposed to be sure of, the more I learn that some of these we cannot be so sure of 100%.