The three modes of knowledge that Rogers identified are subjective, objective, and interpersonal.
Subjective knowledge is that which happens in a person's own mind. Objective knowledge is what is agreed upon as "fact" about reality. Interpersonal knowledge is what individuals know and share about themselves, both their objective and subjective knowledge.
Rogers argues that "a satisfactory science of behavior can emerge only if an appropriate balance between the three modes is obtained." Of all of the schools of psychology, Rogers contends that only humanist psychology achieves this balance.