Any logic differing from classical logic in that there is no presumption that the names used actually refer to anything. Thus from Fa it need not follow that (∃x)(Fx), any more than, in natural language, it follows from the fact that Pegasus is a flying horse that there is such a thing as a flying horse (although an alternative is to say that in the sense in which Pegasus is a flying horse, there are flying horses: i.e. to interpret both statements as implicitly governed by an operator such as ‘in the myths it is said that…’).