reproduce all the superficial peculiarities of an author with such
fidelity that the imitation can hardly be distinguished from the
original: thus few can distinguish between Fenton's work and Pope's
in the translation of the Odyssey. We must take such known facts into
account in deciding a hypothesis of authorship. Such hypotheses can
seldom be decided on internal evidence alone: other circumstantial
evidence--other circumstances that ought to be discoverable if the
hypothesis is correct--must be searched for.

The operation of causes that are manifest only in their effects must
be proved by the same method as the operation of past causes that
have left only their effects behind them. Whether light is caused by
a projection of particles from a luminous body or by an agitation
communicated through an intervening medium cannot be directly
observed. The only proof open is to calculate what should occur on
either hypothesis, and observe whether this does occur. In such a
case there is room for the utmost calculating power and experimental
ingenuity. The mere making of the general hypothesis or guess is
simple enough, both modes of transmitting influence, the projection
of moving matter and the travelling of an undulation or wave movement,
being familiar facts. But it is not so easy to calculate exactly how
a given impulse would travel, and what phenomena of ray and shadow,
of reflection, refraction and diffraction ought to be visible in
its progress. Still, no matter how intricate the calculation, its
correspondence with what can be observed is the only legitimate proof
of the hypothesis.

II.--OBSTACLES TO EXPLANATION.--PLURALITY OF CAUSES AND INTERMIXTURE
OF EFFECTS.

There are two main ways in which explanation may be baffled. There may
exist more than one cause singly capable of producing the effect in
question, and we may have no means of determining which of the equally
sufficient causes has actually been at work. For all that appears
the tares in our wheat may be the effect of accident or of malicious
design: an anonymous book may be the work of an original author or
of an imitator. Again, an effect may be the joint result of several
co-operating causes, and it may be impossible to determine their
several potencies. The bitter article in the _Quarterly_ may have
helped to kill John Keats, but it co-operated with an enfeebled
constitution and a naturally over-sensitive temperament, and we cannot
assign its exac