The All is in all.
'The All' being Absolute Reality and 'all' being life in its individuality or particulars.
The true artist has absolute faith in this truth; that he does no need to force truth from beyond the natural confines of his artistic universe to demonstrate something approaching absolute reality. The general is contained within the particular.
For example, the difference between the devil scenes in Dostoevsky's Brothers Karamazov and Thomas Mann's Doctor Faustus. Mann's scene is interesting for its ideas but wholly unconvincing in artistic terms; one could argue his very eloquently effusive and somewhat stuffy bore of a devil exists in the form necessary to the refined intellectual hero figure but he is little more than a mouthpiece of ideas and doesn't touch the flame of reality. Which isn't to downplay Mann's novelistic talents but his art is very much in thrall to the overarching ideas, and is perhaps more in the way of essay than fiction, or that his fiction is lopsided in the sense of being a clear vehicle for his ideas. Which isn't from my point of view off-putting as Mann found the form most suited to his temperament, and for him to write in a less literary style would be unnatural, and to weaken his actual strengths.
Dostoevsky in contrast says far less but also far more in his much more intriguing scene of Ivan's encounter with his sanity challenging devil. Et cetera.
What is contained within the et cetera is, you will agree, far-reaching and profound, and summing up, the All is not in all in Mann's scene whereas it is in Dostoevsky's.