The basic logic is set forth by Gary North:
Economists will go to almost any lengths to avoid the use of moral terms when they discuss economic issues. This has been true since the seventeenth century, when early mercantilistic pamphlet writers tried to avoid religious controversy by creating the illusion of moral and religious neutrality in their writings. This, they falsely imagined, would produce universal agreement, or at least more readily debatable disagreements, since “scientific” arguments are open to rational investigation. The history of both modern science and modern economics since the seventeenth century has demonstrated how thoroughly unreconcilable the scientists are, morality or no morality.
Nevertheless, traditions die hard. Economists are not supposed to inject questions of morality into their analyses. Economics is still supposedly a “positive” science, one concerned strictly with questions of “if . . . then.” If the government does A, then B is likely to result. If the government wants to achieve D, then it should adopt policy E. The economist is completely neutral, of course. He is just an observer who deals with means of achieving ends. The economist can therefore deal with “complete neutrality,” with this sort of problem: “If the Nazis wish to exterminate 50,000 people, which are the most cost-effective means?” No morality, you understand, just simple economic analysis.The problem with the theory of neutral economics is that people are not neutral, effects of government policies are not neutral, social systems are not neutral, legal systems are not neutral, and when pressed, even economists are not neutral. Because societies are not neutral, the costs of violating a society’s first principles have to be taken into account. But no economist can do any more than guess about such costs. There is no known way to assess the true costs to society of having its political leaders defy fundamental moral principles and adopt any given policy. And if the economists guess wrong—not an unlikely prospect, given the hypothetical moral vacuum in which economists officially operate—then the whole society will pay. (This assumes, of course, that policy-makers listen to economists.)
The inability of economists to make accurate cost-benefit analyses of any and all policy matters is a kind of skeleton in the profession’s closet. The problem was debated back in the late 1930s, and a few economists still admit that it is a real theoretical problem, but very few think about it. The fact of the matter is simple: there is no measuring device for balancing total individual utility vs. total disutility for society as a whole. You cannot, as a scientist, make interpersonal comparisons of subjective utility. The better economists know this, but they prefer not to think about it. They want to give advice, but as scientists they cannot say what policy is better for society as a whole.[1]
This is why politicians and policy-makers have to rely on intuition, just as the economists do. There is no scientific standard to tell them whether or not a particular policy should be imposed. Without a concept of morality—that some policy is morally superior to another—the economists’ “if . . . then” game will not answer the questions that need to be answered. Without moral guidelines, there is little hope of guessing correctly concerning the true costs and benefits to society as a whole of any policy. The economist, as a scientist, is in no better position to make such estimations than anyone else. If anything, he is in a worse position, since his academic training has conditioned him to avoid mixing moral issues and economic analysis. He is not used to dealing with such questions.
And here is Descartes:
“… And although my speculations greatly please myself, I believe that others have theirs, which perhaps please them still more.”