  This paper investigates the significance of T-duality in string theory: the
indistinguishability with respect to all observables, of models attributing
radically different radii to space -- larger than the observable universe, or
far smaller than the Planck length, say. Two interpretational branch points are
identified and discussed. First, whether duals are physically equivalent or
not: by considering a duality of the familiar simple harmonic oscillator, I
argue that they are. Unlike the oscillator, there are no measurements 'outside'
string theory that could distinguish the duals. Second, whether duals agree or
disagree on the radius of 'target space', the space in which strings evolve
according to string theory. I argue for the latter position, because the
alternative leaves it unknown what the radius is. Since duals are physically
equivalent yet disagree on the radius of target space, it follows that the
radius is indeterminate between them. Using an analysis of Brandenberger and
Vafa (1989), I explain why -- even so -- space is observed to have a
determinate, large radius. The conclusion is that observed, 'phenomenal' space
is not target space, since a space cannot have both a determinate and
indeterminate radius: instead phenomenal space must be a higher-level
phenomenon, not fundamental.
