Lots of writing advice goes along the lines of "write for your reader" or "know your audience." That's the guideline for how to frame your writing, what voice to use, and what level of detail to include. 

But what happens if you don't know your audience? Or if your writing isn't meant for a specific group? 

What if you're writing an email to the world, and you don't know who may come across it? ;) 

Then who do you write for?

One of my college professors gave writing advice that I think is more helpful: Assume your reader is intelligent but ignorant. 

Write with the mindset that the reader doesn't know your specific topic, but they're smart and can understand what you're writing. Then it doesn't matter what knowledge or experience they have. You're giving them what they need to follow along. 

That's more useful to me than writing for a specific audience.