The Three Pillars are named (from left to right) the Pillar of Severity, the Pillar of Mildness, and the Pillar of Mercy.  Viewed from this perspective, the glyph of the Tree is presenting to us the fundamental polarity of existence and the path of understanding that resolves it.   Any polar concept -- hot/cold, light/dark, good/evil -- can be assigned to the extreme Pillars of Severity and Mercy.  The secret to the resolution of that polarity is the Tree itself.

The middle pillar may be equated to the Buddha's "middle path," the path of human experience.  Remember the Lightning Bolt of manifestation from the Sephirah text.  It begins at the top of the middle Pillar and then weaves a criss-cross from one polar extreme to the next until it anchors itself into the base of the Pillar.  Never does it pass through Tiphareth or Yesod, the two remaining Sephirah in the middle Pillar; it always strikes the edges of the Tree's expression.  Much like electro-magnetic energy, the power of our life's expression is acquired through the dynamic expression of polar extremes... there is where the realization of energy (and life) is found.

But remember that the Tree expresses a process.  None of the Pillars exist without the context of the others.  The cycle of existence is the realization of the middle path through our experience of the extremes.

We know ourselves through the extremes of our experience.  As children those extremes are fairly close, ranging from a high of winning a spelling bee to a low of losing a beloved pet.  As we acquire more experience, our extremes are extended.  Higher highs and lower lows broaden our understanding of both the joy and the pain of which we are capable.  We derive clarity of being and understanding as we walk the middle path between the two extremes of our experience.  Our awareness and our "knowing"  becomes stronger and with that strength, we find the capacity for love and compassion -- the very qualities of the Creator -- expanding as well.  What better service could we be provided by life than to experience the complete darkness and the radiant light and then know ourselves to greater than either one alone.