definitive design

The author seems to be saying that what people need from an open society has to be frequently re-examined.

(so that it meets the needs of the present)

The way I read it in the context of the sentence, "definitive design" means a society which has been conclusively defined. It therefore cannot be redefined, and hence, ceases to be an open one.

thx~> <

You're welcome

In the process of designing something manufactured, a ship, a washing machine, a plane, whatever, the design will go through a number of revisions, and there comes a stage where they decide that the product is ready to go into production, and the design will be "frozen". The version at this stage is the "definitive" one. Of course human societies are not "designed" in this way.