Paul uses this question to emphasize that no one knows what a person is thinking except the person himself. AT: "No one knows what a person is thinking except that person's spirit" (See: [[rc://en/ta/man/translate/figs-rquestion]])
This refers to a person's inner being, his own spiritual nature.
# no one knows the deep things of God except the Spirit of God
This can be stated in positive form. AT: "only the Spirit of God knows the deep things of God" (See: [[rc://en/ta/man/translate/figs-doublenegatives]])