Of course, there’s the possibility that we help users so that they’ll keep coming back to buy more products and we’ll get paid.

Though technical writers work in areas other than commercial ones, so I suppose those kinds of people work out of some degree of empathy or desire to help.

As you’ve just put it, it appears to me that knowing your audience is a means to an end, and that end is helping the user. Why help the user? S/he doesn’t always need help, but certainly can use the help when technology is not intuitive. This lack of intuitiveness perhaps is the result of a lack of empathy — technology makers can achieve greater capacity more easily than greater usability. It then falls to usability and communication specialists to fill the gap.

This “empathy deficit” might also explain the need to avoid assumptions, and instead to rely on systematic investigation to ensure that we know our audiences and our subject matter well enough to help the two connect.

So, if I were to take a stab at a first principle, I’d say that ingenuity comes to us more easily than empathy. But let’s see how it stands up before accepting it.

Well, I’ll have to start with my opinion here, which is that the objective of technical writing is to make the user successful in whatever his goal is. A corollary to that could be to make the user successful quickly.

Given that, knowing your audience is fundamental because if you know who the user is, what her goals are, what she already knows, what her environment is like, the constraints she’s operating under, and so on, then you know what information to provide and how to structure it so she can accomplish her task and do so as quickly as possible.

If you make assumptions about your user in any of the areas I’ve mentioned or others, you’ll likely reduce the effectiveness of your communication. You’re leaving your user out of the conversation because you’re essentially talking to someone else. When the communication doesn’t connect with the user, she’s less likely to accomplish her aim. You may get lucky and nail your audience, or your audience may be able to interpret what you’re saying into something that makes sense to them. But the first case isn’t likely, and the second case makes more work for the user.

Assumptions also apply to the subject or product you’re communicating about. If you assume things about your subject matter, you may very well mislead the audience, and consequences can range from inconvenience to disaster.

In short, if you don’t know your audience and you make assumptions, your communication is likely to fail to some degree.

What answer would you give to your question?

I’m no expert on logic or philosophy, though; I read about first principles here:

http://en.wikipedia.org/wiki/First_principles

Are you working from a different definition of “first principles”?