"Perhaps the most unique and important principle of the Buddha’s approach to the mind is the insight that the mysteries of the human condition are best explored in the dynamics of subjective experience as it unfolds in the present moment.
Buddhist theoretical psychology is a science of experience, in which the stream of consciousness itself, as it is presented to the attentive and carefully trained observer, is the field of investigation. ... adepts would go off into the forest alone, cross their legs, shut their eyes, and look very closely at what was going on. They would observe the various effects of fasting, breathing exercises, and other yogic disciplines on their experience, and they organized their observations and insights in formal teachings and systems of great subtlety and complexity.
It was a remarkably scientific endeavor in many ways, in which the human body and mind served as the laboratory for investigation. As such, the entire tradition is more of a descriptive phenomenology than a theory of mind.
The Buddha was not saying, 'This is what I theorize human experience to be.' Rather, his message (paraphrased) was, 'This is what I’ve seen in my personal experience.' And further, 'Don’t take my word for it; examine it for yourself, and you too can see exactly what I’m talking about.'
Much of what he points to does not require years in the wilderness to access, but is available to all of us in this very moment."
Andrew Olendzki: "Buddhist Psychology", Chapter 1, in:
Seth Robert Segall ed. "Encountering Buddhism: Western Psychology and Buddhist Teachings", SUNY Press, 2003.