It has not always seemed reasonable to human beings to believe that the natural
world is size layered in both directions. In one direction there was never a prob-
lem. It was obvious even in ancient times that there are objects larger than hu-
man bodies, for the evidence of the human senses testified to the enormous size
of the earth, for example. But that there are also objects smaller than the small-
est objects visible with the naked eye, smaller even than dust motes, was a hy-
pothesis that, for almost 2,000 years from the time of the earliest scientists of
classical Greece, remained a purely speculative belief, supported only by "meta-
physical" arguments of doubtful plausibility.

Fortunately, things do change. During the past 300 years scientists have suc-
ceeded in uncovering more and more fine-grained structure in nature on a smaller
and smaller scale of size. Using artificial devices specially constructed to interact
with this minutely tiny level of reality, practitioners in various fields of inquiry
have claimed to discover the causal mechanisms by which larger scale macro-
scopic phenomena are produced out of smaller scale microscopic phenomena.
This sounds like a simple matter when presented as glibly as I've just now de-
scribed it; but a large number of problems--some methodological, others epis-
temological, and even a few that are ontological--fester at the heart of this
otherwise happy picture of things. Notoriously thorny conceptual problems arise
concerning the nature of the relation between the micro and the macro. To give

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