From 1965 on, Sylvan argued that, through the influence of Quine, contemporary philosophy is committed to a fundamental mistake. Such a mistake, labelled the “Ontological Assumption” (cf. 1980), is represented by the view that one cannot make true statements about what does not exist. In opposition to this idea, Sylvan develops a theory about (non-existent) objects or, in Sylvan’s jargon, items--noneism.
[Noneism is] a very general theory of all items whatsoever, of those that are intensional and those that are not, of those that exist and those that do not (…); it is a theory of the logic and properties and kinds of properties of all items (1980, 5–6)
Sylvan’s intuition is that, in order to be able to say something true about non-existent items, we need to allow for the possibility that non-existent items have properties. For instance, in order to be able to say that ‘Sherlock Holmes is a detective’ is true, Sherlock Holmes needs to have the property of being a detective.
This all seems very straightforward to me (with practically no understanding of ontology), but I take it this view is (was?) rather controversial. So how does Sylvan's view differ from (at the time) mainstream ontological views?