If you're Autistic and I'm Autistic... What even is Autism?

I know that gatekeeping Autism and questioning people's diagnoses is frowned upon and often truly harmful, and yet, I have found myself doing it lately. I think it's very similar to Imposter Syndrome, really. It's all part of this desire to make sense of what it means to be Autistic.

I think, at least for me, it stems from this very strong desire to define Autism. The definitions that exist have always felt rather haphazard to me. We have to fulfill certain criteria, but the criteria are so broad that we can meet them by being polar opposite to one another (hypersensitive to sensory input, hyposensitive to sensory input; overwhelmed by empathy to the point of shutdown, struggling to empathize, etc). I crave a definition that can encompass all of us, but the more Autistic people I encounter, the more I feel like that doesn't exist. Draw the line one way and I'm not Autistic. Draw it a different way and others are not. I will latch onto an explanation, such as the idea that we're all Monotropic, only to realize that some of us aren't. And I really hate the fact that this massive element of my identity is built upon what feels like a flimsy foundation.

Are there any true rules when it comes to Autism, and if there are no actual rules, or no real boundaries, then how can we even know it exists?