The Timaeus of Plato

TIMAIO2. 161
of the visual current and of the object seen come into contact,
the other hand right
contrary to the wonted mode of collision. On
appears as right and left as left, when in the act of combination
with that wherewith it combines the ray changes sides. This
happens when the smooth surface of the mirror is curved upwards
on each side and so throws the right portion of the visual
current to the left side and the ccfoiverse. But if it is turned
lengthwise to the face, it makes this same reflection appear
completely upside down, thrusting the lower portion of the ray
to the upper end and the upper to the lower.
All these things are among the secondary causes which God
uses to serve him in carrying out the idea of the best so far
as is possible. But the multitude regard them not as secondary
but as primary causes, which act by cooling and heating, condensing
and rarefying, and all such processes. Yet they are
incapable of all reason or thought for any purpose. For the
only existing thing to which belongs the possession of reason
moment of impact. In the case of a concave
mirror the section AB would be
a curved line instead of straight ;
and
thereby a ray from the right side, just at
the moment of impact, while it is in act
of amalgamating with the ray from the
object, is shifted to the left side, and vice
versa- It must be remembered that the
concave mirrors of which Plato speaks
are not of the sort with which we are
most familiar, namely hemispherical
mirrors :
they are hemicylindrical : therefore
when the mirror is held laterally, so
that the curvature is from right to left,
the position of right and left as compared
with a reflection in a plane mirror is
inverted ; if it is held vertically (Kara.
pfJKos TOV irpocrwirov), so that the
curvature is from top to bottom, the
reflection is upside down. See Munro's
note on Lucretius iv 317. If the mirror
were hemispherical, or one which is
concave all round from centre to circumference,
both right and left and top and
bottom would be inverted, as may be
seen by simply looking into the bowl of
P. T.
a silver spoon. This case is not noticed
by Plato, nor by Lucretius /. /. Martin
gives a mathematical explanation of the
phenomena.
9. TWV jjvvaiTCwv] Plato now proceeds
to guard against being supposed to
mean that the physical principles which
he has just laid down are the real cause :
they are merely the means through which
the true cause works, viz., poCs operating
^?ri rb (3\Tiffrov. Compare Phaedo 99 B.
The whole of this latter part of the
chapter contains a polemic partly against
Anaxagoras, partly against Demokritos.
Anaxagoras did indeed postulate vovt
as his prime force, but he used it
simply
as a mechanical agent, without attributing
to it a conscious effort to produce the best
result. Demokritos conceives a blind
unconscious force, avdyKi), to be the
motive power of the universe. Thus
whereas the opposition between Demokritos
and Plato is fundamental and
essential, Plato's controversy with Anaxagoras
is due rather to inconsequence or
incompleteness on the part of the latter.
II