Using 莫 in Song dynasty Chinese There is this passage in 语类, where the question of 天地之心 is discussed: 


  所以明道云：‘天地之常，以其心普万物而无心；圣人之常，以其情顺万事而无情。’说得最好。”问：“普万物，莫是以心周遍而无私否？”曰：“天地以此心普及万物，人得之遂为人之心，物得之遂为物之心，草木禽兽接著遂为草木禽兽之心，只是一个天地之心尔。


I find the following sentence in this passage problematic: 


  普万物，莫是以心周遍而无私否


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.
問﹒普萬物﹒莫是以心周遍而無私﹒否


i would interpret it as :

[someone] asked (問): “[about] generalising (普) everything (萬物), can it be that (莫 —> 莫非) [the universal] is using (是以) [its] heart (心) to all over (周遍) and (而) without (無) prejudice (私); isn’t it (否)?


  How to understand this 莫 ?


back to your question, i would say it means “莫非“ (roughly “can it be that”). the asker wanted to ask, but without confidence that his statement is correct, so he inserted the “莫” before the verse “是以心周遍而無私﹒否”; as a safe guard.

clearly, you have fun in reading “朱子語類” :)