The direct use of physical force is so poor a solution to the problem of limited resources that it is commonly employed only by small children and gr… - David D. Friedman

I predict that, if anarcho-capitalist institutions appeared in this country tomorrow, heroin would be legal in New York and illegal in most other places. (p.128)

[T]he assumption describes our actions, not our thoughts. If you had to understand something intellectually in order to do it, none of us would be able to walk.
Economics is based on the assumption that people have reasonably simple objectives and choose means to achieve them. Both assumptions are false-but useful.
Suppose someone is rational only half the time. Since there is generally one right way of doing things and many wrong ways, the rational behavior can be predicted but the irrational cannot. If we assume he is rational, we predict his behavior correctly about half of the tie - far from perfect, but a lot better then nothing. If I could do well at the racetrack I would be a very rich man.
[R]ationality is an assumption I make about other people. I know myself.
[R]ationality is an assumption I make about other people. I know myself well enough to allow for the consequences of my own irrationality. But for the vast mass of my fellow humans, about whom I know very little, rationality is the best predictive assumption available. (pp.3-5)

Economics is that way of understanding behavior that starts from the assumption that people have objectives and tend to choose the correct way to achieve them.