Human spirit
The human spirit was the non-physical part of a person that included their character, feelings, and consciousness.
In 2266, while speaking to Christine Chapel, Brown explained, "Doctor Korby has discovered that as their sun dimmed, the inhabitants of this planet moved underground from an open environment to this dark world. When you were a student of his, Christine, you must have often heard Doctor Korby remark how freedom of movement and choice produced the Human spirit. The culture of Exo III proved his theory. When they moved from light to darkness, they replaced freedom with a mechanistic culture. Doctor Korby has been able to uncover elements of this culture which will revolutionize the universe when freed from this cavernous environment." (TOS: "What Are Little Girls Made Of?")