But what exactly are propositional functions in Principia? This is a matter of heated debate among Russell scholars. Perhaps the most influential interpretation is the constructive interpretation, due to Kurt Gödel (1944). On this interpretation, propositional functions are human constructs of some sort. They depend on our ability to think about them or refer to them. A version of the constructive interpretation can also be found in Linsky (1999). There is also a more nominalist interpretation in Landini (1998). On the realist side, are the interpretations given by Alonzo Church (1984) and Warren Goldfarb (1989). Goldfarb thinks that the logical theory of Principia is motivated by Russell's attempt to find the real nature of propositional functions and that this nature is independent of our thinking about it. Goldfarb has a good point, since Russell's logic is supposed to be a perspicuous representation of the way things are. But Russell often seems to deny that propositional functions are real entities.