SQL and Relational Theory, 2nd Edition

WHY DO WE NEED LOGIC?

Logic is useful because (among other things) it’s a great aid to clear and precise thinking. By contrast, everyone would surely agree that natural language is often vague and ambiguous. The following piece by Robert Graves and Alan Hodge illustrates the point delightfully:^[136]

From the Minutes of a Borough Council Meeting:

Councillor Trafford took exception to the proposed notice at the entrance of South Park: “No dogs must be brought to this Park except on a lead.” He pointed out that this order would not prevent an owner from releasing his pets, or pet, from a lead when once safely inside the Park.

The Chairman (Colonel Vine): What alternative wording would you propose, Councillor?

Councillor Trafford: “Dogs are not allowed in this Park without leads.”

Councillor Hogg: Mr. Chairman, I object. The order should be addressed to the owners, not to the dogs.

Councillor Trafford: That is a nice point. Very well then: “Owners of dogs are not allowed in this Park unless they keep them on leads.”

Councillor Hogg: Mr. Chairman, I object. Strictly speaking, this would keep me as a dog-owner from leaving my dog in the back-garden at home and walking with Mrs. Hogg across the Park.

Councillor Trafford: Mr. Chairman, I suggest that our legalistic friend be asked to redraft the notice himself.

Councillor Hogg: Mr. Chairman, since Councillor Trafford finds it so difficult to improve on my original wording, I accept. “Nobody without his dog on a lead is allowed in this Park.”

Councillor ...