 First, however, let us discuss the extent to which ontological questions are tied to a particular theoretical framework. In its current stage of development, string theory unfortunately provides little indication of the more fundamental nature of space, time, and matter. Despite the consideration of ever more exotic objects — strings, p-branes, D-branes, etc. — these objects are still understood as propagating in a background spacetime. Since string theory is supposed to describe the emergence of classical spacetime from some underlying quantum structure, these objects are not to be regarded as truly fundamental. Rather, their status in string theory is analogous to the status of particles in quantum field theory (Witten, 2001), which is to say that they are relevant descriptions of the fundamental physics only in situations in which there is a background spacetime with appropriate symmetries. While this suggests tantalising links to issues of emergence, it is difficult to pursue them without knowing the details of the more fundamental theory. As already mentioned, the duality relations between the various string theories suggest that they are all perturbative expansions of some more fundamental, non-perturbative theory known as ‘M-theory’ (Polchinski, 2002, see the Other Internet Resources section below). This, presumably, is the most fundamental level, and understanding the theoretical framework at that level is central to understanding the underlying ontology of the theory (and so the manner in which any other structures might emerge from it). ‘Matrix theory’ is an attempt to do just this, to provide a mathematical formulation of M-theory, but it remains highly speculative. Thus although string theory purports to be a fundamental theory, the ontological implications of the theory are still very obscure — though this could be viewed as a challenge rather than a reason to ignore the theory.