"the" rate of profit cannot fall. Thus, even Duncan K. Foley, a prominent critic of the TSSI, acknowledges that "I understand [Alan] Freeman and [Andrew] Kliman to be arguing that Okishio’s theorem as literally stated is wrong because it is possible for the money and labor rates of profit to fall under the circumstances specified in its hypotheses. I accept their examples as establishing this possibility.” In 1906-07, Ladislaus von Bortkiewicz claimed to prove that Marx's account of "the transformation of commodity values into prices of production" (i.e., prices that allow businesses to obtain an average rate of return on their capital investments) was internally inconsistent. "Correcting" the inconsistency, Bortkiewicz produced results that seriously call into question Marx's theory that prices and profits are determined, in the aggregate, by the production of value and surplus-value: the "price rate of profit" no longer equals the "value rate of profit", and the sum of prices diverges from the sum of values. However, proponents of the TSSI claim to have disproved Bortkiewicz's position. Although no critic has successfully refuted this claim, first put forward in 1988, many Marxist economists still decline to accept it. When Marx's theory is understood in accordance with the TSSI, rather than in accordance with Bortkiewicz's interpretation, moreover, the results of his transformation account re-emerge as internally consistent; price and value magnitudes are indeed equal in the aggregate. These equalities also re-emerge under other––atemporal––single-system interpretations. Yet under the atemporal interpretations, Marx's falling-rate-of-profit theory and other aspects of his value theory still appear to be internally inconsistent. In order for his theories to be fully acquitted of charges of inconsistency and error, he must be interpreted as having had a temporal conception of value and price determination. Criticism Critics of the TSSI have characterized it as an outdated theory which asserts that "Marx made no errors". For instance, David Laibman charges that its proponents are non-scientific Marxists who "assert that Marx's formulations, in both the theory of value and the analysis of capitalist accumulation and crisis, are literally and completely correct; that Marx made no errors . . . ." Roberto Veneziani similarly alleges that the TSSI upholds "the literal truth of all [of] Marx’s propositions". Proponents of the TSSI contend that these allegations are false:"We have never said that Marx’s contested insights are necessarily true . . . . We simply say the claims that his value theory is