An immediate reply is that the illegitimate generalization L4 involves a temporal restriction, just as L2 was restricted spatially (see e.g., Carnap 1947). The idea would be that predicates that cannot be used for induction are analytically “positional”, i.e., their definitions refer to individual constants (for places or times). A projectible predicate, i.e., a predicate that can be used for induction, has no definition which would refer to such individual constants but is purely qualitative (e.g., because it is a basic predicate). The trouble is that this reply makes it relative to a language whether or not a predicate is projectible. If we begin with a language containing the basic predicates “green” and “blue” (as in English), “grue” and “bleen” are positional. “Bleen” is defined as follows: