this is interesting.In the series of contemplations outlined, the meditation on boundless space is a critical link. Space is, technically, still within the purview of the material. In other words, it is a physical property, whereas Mind, the next stage, is not. And yet Space shares some characteristics with the formless - it is empty, boundless and insubstantial. In the Sarvastivada Abhidharma, space is classed as unconditioned, although both Theravada and Einstein would disagree.
*how* does one go from focusing on bit parts of the body (and sensations) to something like boundless space? (which is where this gets waaay too spacy for me)
it seems to *invite* a loss of focus...
if i can't perceive it, should i imagine it?
The switch from meditation on earth (and earth element) to space is done like this; visualize earth-element, gradually extending your field of awareness until you are contemplating the whole of this planet (paying attention only to earth; never mind the pesky little things crawling around on the surface). Hold this whole globe of solid, impenetrable earth-element in your consciousness for a while. Now - remove it and pay attention only to the space that the earth takes up.
Since space has, by it's nature, no boundaries this should naturally dissolve into boundless space.
Remember, this series is accomplished by progressive selective non-attention. Space is what is left when you remove earth. That's really all there is to it.
Ajahn Sumedho sometimes talks about trying to be aware of the space in a room, as opposed to the objects. This is done with a kind of foreground-background switch and can help to get the "feel" of contemplation of boundless space also.