"Enter eagerly into the treasure house that lies within you, and so you will see the treasure house of heaven...The ladder that leads to the Kingdom is hidden within you." sixth century Christian mystic St. Issac the Syrian
I'm faced with this problem:
I teach, mostly, asana.
I tell people the asana don't really matter; yoga begins with a desire to wake up, with ethics and personal observance, with self study and commitment.
And I tell them the way is through asana.
You see the problem. I'm contradicting myself.
We all walk into yoga knowing it has to do with postures. A few of us figure out, along the way, that yoga has nothing to do with postures. If we've made the commitment to a regular practice, if we keep knocking on that door of the body, if we practice when we are tired and when we've not slept well and when we don't want to, practice when we're too busy and when we're happy enough without it, eventually we feel joy come to answer the knocking. Joy erupts as deeply as an orgasm and as incorrigibly as age. Grace only ever happens in real time.
But those things - ethics, commitment, self study - remain abstract for most humans. Asana offer a discipline, an opportunity, a path. Maybe a ladder. They give a way in to meditation, to healing, and to the present moment. Most of us wouldn't have the guts or the time to get there on our own. Asana is the teacher, is the commitment. Asana is the guru.
We show up and are prodded into the present moment.
The body lives in the present. When you are aware of the body, you are connected. To what I won't bother to say. Maybe the global throb of life. The on-goingness of it. The truth of dailyness. Eternity. God. An underlying okayness. The realization of how small and irreal your hang-ups are, considering reality. How big they are, as hang-ups. The present, via the body moving and the mind watching, will reveal the stories you tell yourself day in and day out. If you manage to trace edges with your breath and your toes, the present will prove to you that these stories are untruths. Half truths at best. Signals of compromise. Misunderstandings. 
The present, via the body, is the one place from which you can see reality. Awareness of the body is our gateway into the truth of what is.
Pema Chodon writes "To be fully alive, fully human, and completely awake is to be continually thrown out of the nest. To live fully is to be always in no-man's land, to experience each moment as completely new and fresh. To live is to be willing to die, over and over again."
Asana throw us directly into no-man's land.
I think that is where we need to be.
***
The simplest explanation for why, in the eight limbed path, there are asana is this: if you want to reach the inner self, you have to go through the self. It is hard to feel alive, let alone awake, if you are stuck in a body that is unwell. If you want to go the depths of who you are and what you are capable of, it helps if your most immediate and constant tool - ie flesh and blood - become resource rather than hindrance.
It is hard to find reality if you are unaware of your own heartbeat.
So the body itself becomes an object for meditation. The body itself is medicinal, therapeutic. Asana provide a genuine high and a refuge. Asana gives us a place to go. It lays out pathways and intricacies of mastery and skill. They strengthen and sooth, open and release.
But there is something more than the simple explanations.
If we can manage to show up in the body, to drop in, we experience. We feel something. Something is known that wasn't known before.
Because it is body - or whatever it is that is real inside and outside the body - it is not a thing of the mind. Language can only approximate it. Like love, asana is a thing that has to be experienced, rather than talked about. Also like love, asana is expressed in metaphor and poetry. It involves ecstatic release, profound rest, changed brainwaves. Like love, the entirety of the experience can never be understood from the outside.
But we've touched something. It's eerie at times. The fact that there is something there. To reality. To body. This isn't necessarily what we came looking for.
***
A deeper understanding reveals itself. Our brain is everywhere the nerves go. Heart is everywhere the blood is. The practice of asana teaches fairly quickly that our bodies are much more complex, or perhaps more stiff, than we'd known. What we took for granted, as reality, as limitation, proves to be conditioning or simply a process we haven't completed, yet. It also teaches, in little shivers of recognition, that we can know our bodies more profoundly. Where body is, mind and heart and emotion can go. Meditation and awareness can go deeper. What was unconscious in us is brought closer.
If the simple reason for asana is clarification and refinement of the body, the more complex reason is the fact that bodies are our most direct route to reality and its depths. Deep involvement and attention to asana brings us directly to (perhaps, perhaps...through...) mind and it's shadows. You can't work physical patterns very long without banging smack up against psychological patterns. 
You cannot practice asana for long without having to acknowledge that even mind and emotion, urge and insight, knowledge and clarity, are more profound and shadowy than you thought. In the deep silence underlying your breath, you'll recognize you're facing a door. To enter possibility, to to turn away. The pose begins exactly as you most want to leave it.
***
Psalms, songs, beatitudes, and prayer are all words that come to mind when I try to write about asana. There seems to be no more literal way to commune than to examine what it is we do with our hands, or to open our heart. To touch gratitude, acceptance, dedication. It is one thing to understand such concepts. Another, deeper thing, to embody it.
I teach asana. Sometimes I can hear resistance and disbelief roaring out of my students bodies. My foot, where? The hell you say. I practice asana, and I hear that same roar inside myself. Here I am, lurching through no-man's land, all over again. But it has been in asana, in that very place of disbelief and breath, that revelation comes. There have been times I seem to break through in a pose I've done for years; the body shifts a millimeter, perception gets brighter, it seems there is bliss inside the hamstring. There have been other times, crumpled on the mat with my knee no where near where it's supposed to be, that fear has been revealed. Or longing. Absolute surrender and behind the surrender the sensations which are moody and pithy and cogent and altogether sweet. There's the thought I didn't know I could feel this. There are poses, too, that I have doggedly practiced - without success - for months and months and years on end without much believing I'll ever truly get there. When suddenly, I am there. The foot lifts. The rib moves out of the way. The heart stretches.
We have potential in our gristle. The root truth is this: if we experience pleasure, pleasure is experienced through the body. If we experience fear, grief, or longing, it is because our physicality has been shifted and touched in fine or blatant ways. If we honestly desire health, wellbeing, contentment, it must involve the chemistry and patterns of hormones, digestive proteins, cellular structures. If we have ever longed for god, or felt our heart clutch in some manner of loneliness, it has been a physical pang. Therefore, we come closer by going through. We bend back on our selves, attention revolved back toward itself, the body a mirror in which we can begin to see.
Asana is a dedicated form by which we turn the abuse and denial of the body back into humility, feeling, and meaningful gesture. Asana is how we turn our bones to dancing, our wrinkles to poems. Asana is a psalm made of flesh and bone.