impressive has been left by the historian Thucydides, himself not only a
spectator but a sufferer. It is not one of the least of his merits, that
his notice of the symptoms, given at so early a stage of medical science
and observation, is such as to instruct the medical reader of the
present age, and to enable the malady to be understood and identified.
The observations with which that notice is ushered in deserve particular
attention. "In respect to this distemper (he says), let every man,
physician or not, say what he thinks respecting the source from whence
it may probably have arisen, and respecting the causes which he deems
sufficiently powerful to have produced so great a revolution. But I,
having myself had the distemper, and having seen others suffering under
it, will state _what it actually was_, and will indicate in addition
such other matters as will furnish any man, who lays them to heart, with
knowledge and the means of calculation beforehand, in case the same
misfortune should ever occur again."

To record past facts, as a basis for rational prevision in regard to the
future--the same sentiment which Thucydides mentions in his preface, as
having animated him to the composition of his history--was at that time
a duty so little understood that we have reason to admire not less the
manner in which he performs it in practice than the distinctness with
which he conceives it in theory. We infer from his language that
speculation in his day was active respecting the causes of this plague,
according to the vague and fanciful physics, and scanty stock of
ascertained facts, which was all that could then be consulted. By
resisting the itch of theorizing from one of those loose hypotheses
which then appeared plausibly to explain everything, he probably
renounced the point of view from which most credit and interest would be
derivable at the time. But his simple and precise summary of observed
facts carries with it an imperishable value, and even affords grounds