Relevant to the topics of spirituality and psychology is the concept of epistemology. Epistemology deals with how a person knows that something is true, i.e., what evidence a person accepts as proof that a particular thing exists.
Some spiritual disciplines ascribe to the belief that truth is known through direct experience. A Christian might believe in God because he has seen a person healed through prayer (although others might not except this person's healing as proof that God exists) or because he has felt the "holy spirit" move through him. Buddhists believe that one can eventually experience his or her true essence through the continued practice of mindfulness meditation. Whenever this practice is explained the person doing the explaining always cautions that the experience of one's true essence cannot be adequately described; one has to experience it directly in order to understand it.
Also prominent among spiritual disciplines is faith epistemology, i.e., "I believe in something; therefore, it is true."
Psychology, in its struggle to be accepted as a "real science" (as opposed to a "pseudo-science"), also looks to direct experience as proof that something exists. However, the field of psychology will not except subjective experience; proof that something exists must be demonstrated objectively via testable hypotheses and observable results (that can be duplicated). In short, psychology has moved towards relying on the scientific method to determine what is true.
In doing a bit of background research I found that epistemology is a very complex subject (philosophical in nature) with numerous types and subtypes. What struck me was that none of the types and subtypes I read about seemed to be sufficient in and of themselves; it does not appear that any one method is the absolute and correct way for determining what is true. There are different types of truth and many ways of knowing. Truth cannot always be reduced to "either-or;" frequently we must think of it as being "both-and."