Someone has a vision of God. Or feels one with the universe. Or has a near-death experience that gives them a glimpse of heaven. Or comes to know that Jesus loves her.
What should we make of such experiences?
They are undeniably subjective. Yet so is everything that we humans experience as conscious beings. I have no direct access to the consciousness of any other person, nor does anyone else have access to mine.
There are two extreme answers to the question I posed.
One is to make subjectivity unquestionable. If somebody says "I've seen God!" no one else has a right to challenge them, or look into the basis for their subjective experience. The other answer is to deny subjectivity, making truth a matter of purely objective evidence that can be rigorously substantiated.
I like the middle way espoused in Stanislas Dehaene's book, "Consciousness and the Brain: Deciphering How the Brain Codes Our Thoughts."
Dehaene, a neuroscientist, says that researchers into human consciousness need to take subjectivity seriously. There is a big difference between the brain experiencing something unconsciously, and consciously.
All this evidence points to an important conclusion, the third key ingredient in our budding science of consciousness: subjective reports can and should be trusted.
...Whatever information we are conscious of, we can name it, rate it, judge it, or memorize it much better than we can when it is subliminal. In other words, human observers are neither random nor whimsical about their subjective reports: when they report an honest-to-god feeling of seeing, such conscious access corresponds to a massive change in information processing, which almost always results in an enhanced performance.
But subjectivity shouldn't be accepted unquestioningly. People can be wrong, and often are, about the reality of what they experience.
This being said, we should not be naïve about introspection: while it certainly provides raw data for the psychologist, it is not a direct window into the operations of the mind. When a neurological or psychiatric patient tells us that he sees faces in the dark, we do not take him literally -- but neither should we deny that he has had this experience.
We just need to explain why he has had it -- perhaps because of a spontaneous, possibly epileptic activation of the face circuits in his temporal lobe.
Even in normal people, introspection can be demonstrably wrong. By definition, we have no access to our many unconscious processes -- but this does not prevent us from making up stories about them... Consider what happens when we try to make sense of our past actions. People often invent all sorts of contorted, after-the-fact interpretations for their decisions -- oblivious to their true unconscious motivations.
...However, as a measure, introspection still constitutes the perfect, indeed the only, platform on which to build a science of consciousness, because it supplies a crucial half of the equation -- namely, how subjects feel about some experience (however wrong they are about the ground truth).
To attain a scientific understanding of consciousness, we cognitive neuroscientists "just" have to determine the other half of the equation: Which objective neurobiological events systematically underlie a person's subjective experience?
Dehaene goes on to offer the example of near-death experiences. Some surgery patients report that they leave their bodies during anesthesia and are able to look down upon their inert body from on high. He asks, "Should we take them seriously? Does out-of-body flight 'really' happen?"
Experiments have shown that stimulating a part of the brain can produce an out-of-body experience. So...
Out-of-body flight "really" happens, then -- it is a real physical event, but only in the patient's brain and, as a result, in his subjective experience. The out-of-body state is, by and large, an exacerbated form of the dizziness that we all experience when our vision disagrees with our vestibular system, as on a rocking boat.
So, yes, every honestly reported subjective experience is real, because someone had the experience. But only by knowing what produced that experience, the neurological correlates, can we assess whether it was "merely" subjective or a reflection of objective reality.
Along this line, David Lane and Andrea Diem-Lane have written an interesting essay, "The Material Basis of Near-Death Experiences." Check it out. The essay addresses a similar theme: what should we make of a subjective claim about supernaturalism, when it can be explained by natural causes?