Note: the text of this document originally appeared as a Usenet post to the writing newsgroup, misc.writing.
Unlike Western poetry, there is no division between form and content in haiku. This all starts with the inspiration for haiku and ultimately, its content - an aesthetic moment. Aesthetic moments are those flashes of inspiration and knowing that images can produce. During such moments, the boundaries between subject and object are blurred and, if you're lucky, completely erased. You are no longer an observer; you are simply existing in an experience. This is the reality of "being one with nature." This is what it's all about.
So, how is an aesthetic moment tied to the form and content of haiku? Well, according to some schools of haiku, the haiku form is dictated by the length of an aesthetic moment. And that length is the lifespan of a single breath. Let's use an actual haiku to illustrate this.
yellow leaves
on branches overhead --
now falling
In this case, the experience is that very moment when the leaves on a tree are released and fall to the ground. You are there, under the tree, watching that transition. How long does that experience last? You exhale and it's all over. The moment has passed.
Because the haiku experience is short-lived, the form must reflect that. Linguistically, the number of syllables most comfortably uttered in a single breath ranges between 15 and 18 [1]. Thus, the form of the haiku is created by the experience.
And so, form and content are inextricably connected in haiku.
[1] Note: I would argue that, given the varying lengths of syllables in the English language due to differing vowel lengths, etc., English language haiku is best suited to a fewer number of syllables.