A recent thread about the platypus from @vagina_museum gave a great example of biological realities refusing to conform to categories humans delineate and of our scientific understanding of reality evolving when new evidence is presented.
I'd like to expand on how this actually works when we conduct research.
The scientific endeavour is predicated upon our ability to accept new information that goes against the current understanding and come away with a more complete understanding of how things actually are.
This does not mean that at any point we fully understand anything.
New information will always arise with advances in technology, discoveries, and people entering the field.
Science: "All mammals give birth to live offspring." Reality: "Here's a platypus." Science: "Hmm. Okay, most mammals give birth to live offspring, but not monotremes."
This is why the scientific method doesn't actually involve proving anything; we can't know what new information will come to light.
Instead, we propose a hypothesis that can be tested and proven false. We try to prove it false. If we succeed, the hypothesis was wrong.
However, if we fail to prove our hypothesis false this does not mean it is true. It means we have failed to prove it false.
New information could come to light later—a new way of measuring something relevant, the existence of an organism we haven't seen before, et cetera.
We advance science by incorporating new information into the formulation of new hypotheses.
For the moment, our understanding is that all mammals produce milk. Even the more unusual ones, like monotremes and marsupials.
But could that change? Absolutely!
Biology doesn't care about the characteristics humans decide are inherent to any particular category. We decide them based on evidence available to us, but there's no guarantee that evidence represents the entirety of reality.
And that's okay! So long as we recognise that.
Problems arise when humans insist the historical categorisations of individuals are correct. Especially categorisations of humans.
In the worst scenarios, it leads to divisiveness, 'othering' of entire groups based on spurious evidence, racism, sexism, bigotry of all kinds.
Rigid adherence to existing ideas—even ones that have come from the scientific method in the past—is just about the least scientific attitude someone can have.
Adaptability and evolution of our processes, ideas, and how we view the world is the only way science can further our understanding of reality.
But the truth isn't something we'll ever reach. Our understanding will get better, but there'll always be information we lack.