  The causal structure of a strongly causal spacetime is particularly well
endowed. Not only does it determine the conformal spacetime geometry when the
spacetime dimension n >2, as shown by Malament and Hawking-King-McCarthy
(MHKM), but also the manifold dimension. The MHKM result, however, applies more
generally to spacetimes satisfying the weaker causality condition of future and
past distinguishability (FPD), and it is an important question whether the
causal structure of such spacetimes can determine the manifold dimension. In
this work we show that the answer to this question is in the affirmative. We
investigate the properties of future or past distinguishing spacetimes and show
that their causal structures determine the manifold dimension. This gives a
non-trivial generalisation of the MHKM theorem and suggests that there is a
causal topology for FPD spacetimes which encodes manifold dimension and which
is strictly finer than the Alexandrov topology. We show that such a causal
topology does exist. We construct it using a convergence criterion based on
sequences of "chain-intervals" which are the causal analogs of null geodesic
segments. We show that when the region of strong causality violation satisfies
a local achronality condition, this topology is equivalent to the manifold
topology in an FPD spacetime.
