Boole’s Definition of universe of discourse

“In every discourse, whether of the mind conversing with its own thoughts, or of the individual in his discourse with others, there is an assumed or expressed limit within which the subjects of its operation are confined.

The most unfettered (loose) discourse is that in which the words we use are understood in the widest possible application, and for them the limits of discourse are co-extensive with those of the universe itself.

But more usually we confine ourselves to a less spacious field.

Now, whatever may be the extent of the field within which all the objects of our discourse are found, that field may properly be termed the universe of discourse. Furthermore, this universe of discourse is in the strictest sense the ultimate subject of the discourse.”