While every group has certain economic interests identical with those of all groups, every group has also, as we shall see, interests antagonistic to those of all other groups.

While certain public policies would in the long run benefit everybody, other policies would benefit one group only at the expense of all other groups.

The group that would benefit by such policies, having such a direct interest in them, will argue for them plausibly and persistently. It will hire the best buyable minds to devote their whole time to presenting its case. And it will finally either convince the general public that its case is sound, or so befuddle it that clear thinking on the subject becomes next to impossible.

In addition to these endless pleadings of self-interest, there is a second main factor that spawns new economic fallacies every day.

This factor  is the persistent tendency of people to see only the immediate effects of a given policy, or its effects only on a special group, and to neglect to inquire what the long-run effects of that policy will be not only on that special group but on all groups. It is the fallacy of overlooking secondary consequences.

In this lies the whole difference between good economics and bad.

The bad economist sees only what immediately strikes the eye; the good economist also looks beyond.

The bad economist sees only the direct consequences of a proposed course; the good economist looks also at the longer and indirect consequences.

The bad economist sees only what the effect of a given policy has been or will be on one particular group; the good economist inquires also what the effect of the policy will be on all groups.

The whole of economics can be reduced to a single lesson, and that lesson can be reduced to a single sentence:

"The art of economics consists in looking not merely at the immediate but at the longer effects of any act or policy; it consists in tracing the consequences of that policy not merely for one group but for all groups."