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**naive (intuitive) definition of "set"**

I happened upon a book by a Joseph Landin, once head of the math department at University of Chicago and subsequently Ohio State University, in which he gives this as a definition of a set and states this property:

A set is a collection of objects; the nature of the objects is immaterial. The essential characteristic of a set is this: Given an object and a set, then exactly one of the following two statements is true.

a) The given object is a member of the given set.

b) The given object is not a member of the given set.

Shortly thereafter, he writes,

It might be tempting to speak of "the set of people who will visit the city of Chicago during 2050." But, clearly, such a collection cannot qualify as a set according to our understanding of this term.

Would you please explain why his second statement is so? I cannot fathom why this is not a perfectly consistent and constructible set.