It refers to common sense. It is a philosophical notion of inborn ideas that are common to all of us who were born into humanity. Aristotle was the first one who coined the term, but it would also be used in the Enlightenment philosophy, which claimed that humans are born neither good nor bad. Their attitudes, behavior and morals are formed by society; yet, they do have something in common, something that precedes education - and that is common sense. To paraphrase Locke, it is an innate principle or primary notion that precedes knowledge.
