The philosopher has forgotten that he is part of nature and is born of nature. He tries to answer questions by his lonesome self, all in his mind.
To built a dam to block water. It is knowledge if water is actually damned up. To plant seeds. It is knowledge if the field grows to a tall golden harvest.
The Criterion is simple: if it works it's knowledge; if it does not work it's still knowledge. All these in the context of nature into which the philosopher is born and makes his observations and interactions.
Nothing circular here nor is vicious regress necessary. It is knowledge because you observe it in nature and interact it with nature.
Good Grief.
