Emerson

In considering the influence of nature on the scholar, Emerson relates his view, in the first pages of his Oration, that nature and the scholar proceed from one root in a way that nature becomes the limit for the scholar's fulfillment. So "Know thyself" and "Study nature" are taken to be one maxim.
In the following section, where he considers the influence of books, he explains that even though the "first" scholar first experienced and then produced his work, no artist, including scholars, can avoid the conventional entirely. Thus every age needs to write or produce its own works. In other words, to become Man Thinking we need to begin from our values, principles, or point of view. The book, quite the opposite, deals exclusively with some genius of the past and thus kills creativity. For creativity he reminds us, in the form of manners, actions and words, is indicative to no authority or custom. Nevertheless, he is not willing to burn all books in the name of creativity. But at the same time he reveals that creative reading is to gather from each book those few "authentic utterances of the oracle". What he means by this, is that since the scholar can only learn from nature, by action as we'll see, there is only one truth. A truth which we can see with the active soul.
In what follows Emerson goes on to praise action since for him, the attractions of the world, nature, unlock his thoughts and reveal himself to him. Experience's product is nothing but thought. It is as simple as "The more you put into it the more you get out of it". Thus Emerson sees the value of action a resource.
Therefore, what Emerson is saying in the assigned passage, is that in the case where an artist has captured a truth, that truth is not his exclusively. To learn this truth from the artist instead of experiencing and producing this truth is thus to learn from a delegate what one can do, that is, to know, on his own.