There is this passage in 语类, where the question of 天地之心 is discussed:
I find the following sentence in this passage problematic:
How to understand this 莫 ? Does it relate to 万物 or to 天地之心?
My intuitive understanding of this is that it points to the fact that there is nothing left of the Heart/Mind to be personal Heart/Mind of Heaven and Earth (because it is all everywhere). But it is more a guess than a genuine understanding.