[Originally published in The Voluntaryist]
The Voluntaryist: What is the significance of emphasizing that our subjective preferences are ordinal and not cardinal?
Jessica Carswell: It means that you can't make interpersonal utility comparisons and therefore can't objectively prove that any forced exchange or redistribution is for the "common good". To do that, you'd have to be able to measure total utility: i.e. utility gained minus utility lost. To measure, utility would have to be cardinal.
It also explains why people exchange. Because each party ranks the goods or services received more than the goods given up. Which has implications on where prices come from - that it's subjective value, not some "objective" or inherent value that set prices.
The fact that you can't make interpersonal utility comparisons is basically the case against the state in its entirety. Even if you want it to do "good" things through violent intervention/redistribution, you can't prove objectively that these things are "good" overall.
You might personally believe that it's "moral" to steal from the very rich to help the very poor, or to violate someone's rights to some small extent to prevent a severely bad outcome (e.g. borrowing a gun against a person's will to prevent a mass murder). But you can't prove objectively or scientifically that by doing so you are achieving a common good outcome. These are win-lose cases and you can't objectively measure the win against the lose to ascertain whether you have a positive net outcome.
The Voluntaryist: Beautiful response which basically refutes socialism and the labor theory of value in short order. So, to get it straight, viewing rankings as cardinal would allow for a sort of utilitarian common-good justification for theft in seeing Person A valued his 1st-ranked preference only 1.1x more than the 2nd, and so therefore it would be just and moral, and not a loss of "social utility", then, to transfer this to Person B who supposedly values it more, and therefore this is why preferences must be seen as ordinal – 1st, 2nd, 3rd – as opposed to 1st being valued 1.5x more than the 2nd, and so forth? Is that a way to put it?
Jessica Carswell: Yeah basically cardinal utility would allow for some kind of objective measurement. Ordinal does not.
It's not an assumption. Preferences are ordinal.