Not a native Chinese speaker, but I am a proficient speaker of Chinese as a second language. No dog in this fight except to mention that "天真" does have a "sum of its parts" aspect for me. I sometimes think of it in relation to the Chinese word for "congenital" (天生, tiānshēng). In the case of congenital, the 天 (tiān) part is better translated as "heaven", "God", "fate" or "nature", and for me, carries aspects of all of those English words. 生 (shēng) in this context means "born" as in "was born with". Think "congenital defect", a defect that you had before birth, which you could only blame God, or nature for.
So 天真 (tiānzhēn), along the same lines, roughly translated, means "then sense of reality that you have when you are born or which you are gifted by nature", unsophisticated and naive. Don't know if that makes sense, but I've always thought about these two words together and felt like I understood them better through context.
