… is from page 379 of Liberty Fund’s newly published, expanded English-language edition , expertly edited by David Hart, of Frédéric Bastiat ’s indispensable work Economic Sophisms and “What Is Seen and What Is Not Seen” ; specifically, it’s from Bastiat’s March 1848 essay “A Disastrous Remedy” (“Funeste remède”):
[I]t is not the goodness of the intention that makes the goodness of the medicine. A mortal remedy can be given in all charity.
DBx: Bastiat here expresses one of the core insights of economics: intentions are not results, and results are not determined exclusively – or in many cases  even remotely – by intentions.
When stated in isolation this insight appears to be trivial. Indeed, it seems to be too obvious for words. And yet many people regularly forget the truth of this insight when judging the merits of their favorite politician’s, pundit’s, preacher’s, pope’s, or professor’s latest proposal to use government force to improve humanity.
Less common is an error that some economists slip into – namely, mistakenly assuming that anyone who intends to do good – that is, who intends to improve the well-being of others – will generally wind up doing unintended harm. While a great many good intentions are indeed pregnant with unintended bad outcomes, it does not follow from this fact that good outcomes emerge only from self-interested intentions. Good outcomes can result from good intentions. And not all good outcomes are the unintended results only of narrowly self-interested intentions.