Where nature and nurture intersect to create human behavior is as evasive a locality as where the physical brain and metaphysical consciousness intersect to create human thought, if such an intersection exists at all. The discussion of just such an intersection has occupied much of philosophy past, psychology past and present, and anticipates the biopsychology of the future. Thus, implicit in the discussion of this intersection is an understanding of the realm of mind and consciousness, how each is related to the physical body, and the consideration of an independent consciousness that transcends the physical functions of the body.
If a tree falls in the forest and no one perceives it, did the tree really fall? For the ardent empiricist George Berkeley the answer is a resounding "yes". Berkeley believed that an objective reality does not exist apart from our perception of that reality (Goodwin, 2005). This puts Berkeley at odds with the materialist tradition of his time, which asserted that all that exists is objective reality. For him, the act of perceiving was the reality. The way he circumvented the aforementioned question, reminiscent of Einstein's cosmological constant, is that God, the Permanent Perceiver, is omnipresent and can therefore perceive every event concurrently. According to Berkeley the mind existed only as an extension of consciousness, rather than the other way around, a concept that has been labeled subjective idealism. In contrast, Gottfried Leibniz proposed that the physical and metaphysical work in concert, parallel in fact, held constant by the direct hand of God himself, a notion that Leibniz stated as psychophysical parallelism. To apply Leibniz to the above question, the tree did fall because an objective reality, apart from our perception of it, exists. He bridged the gap between mental and physical reality through the use of what he called monads. Leibniz hypothesized that through the energy forces of monads consciousness could exist in the physical mind. He went on to conclude that certain mental processes operated below the level of perception and called them petites perceptions. This further insight would lay the groundwork for the Freudian understanding of unconscious mental processes.
The Enlightenment brought with it the move in intellectual circles from a dependency on immaterial explanations of human consciousness to the belief that objective reality could be understood through scientific methodology. A last vestige of the Renaissance and the legacy of immaterialism was the vitalism of Johannes Muller. Muller proposed that in addition to the physical and chemical machinery of physiological systems a "vital force" underlay mechanical and mental action. Vitalism predicted that a special life force, which could create its own energy, could explain the link between the physical