 This systematization is based on the assumption that there is a common factor in seeing and suffering from a visual illusion: both are experiences of a state of affairs, the only difference is that the states of affairs does not obtain in the second case. McDowell (see his 1982) and other philosophers argue that it is a mistake to hold that veridical perception and illusion involve a common factor. The plausibility of an argument for states of affairs as the primary objects of perception depends on the defensibility of the common factor assumption.