Common knowledge is knowledge that has been acquired and formulated by more than one author. Such knowledge should by default not be permitted to become private property.
An author who quotes other authors should get only the honor part of their work, not the revenue part (compare There are two aspects of copyright). The revenue part, the right to share this knowledge, should be a human right, freely granted to everybody.
For example when an end user of a software asks a question about how to use this software, then she actively contributes to the knowledge pool around that software.
Note the important relation between common knowledge and private data. Public data should be usable, private data should be protected 1. Disclosing confidential data to the public is a criminal act which cannot be undone.