Propertarians talk a lot about reciprocity. And rightfully so.
Reciprocity is the exchange of roughly equivalent value between consenting people. If you're benefiting from the efforts of others without reciprocating in kind, you may be a parasite.
But in my experience, they don't seem to talk about the fact that value is subjective.
Value equivalence requires a subjective analysis and thus cannot be objective and thus also cannot be "scientific".
Markets are how we calculate prices, but markets are simply machines of subjective value analysis.