“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”, says David Friedman in Price Theory: An Intermediate Text describing the concept of the homo economicus in a simple way.
What is interesting to me is that Friedman leads to a point a critic cannot possibly deny: To assume, that people try to act rational. And he is going even further: Acting rational is the only adequate assumption one can do. Friedman in Hidden Order: The Economics of Everyday Life:
“The 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 time – far from perfect, but a lot better then nothing.”