We have now explained what the transparent is and what light is;
light is neither fire nor any kind whatsoever of body nor an efflux
from any kind of body (if it were, it would again itself be a kind
of body)-it is the presence of fire or something resembling fire in
what is transparent. It is certainly not a body, for two bodies cannot
be present in the same place. The opposite of light is darkness;
darkness is the absence from what is transparent of the
corresponding positive state above characterized; clearly therefore,
light is just the presence of that.
Empedocles (and with him all others who used the same forms of
expression) was wrong in speaking of light as 'travelling' or being at
a given moment between the earth and its envelope, its movement
being unobservable by us; that view is contrary both to the clear
evidence of argument and to the observed facts; if the distance
traversed were short, the movement might have been unobservable, but
where the distance is from extreme East to extreme West, the draught
upon our powers of belief is too great.
What is capable of taking on colour is what in itself is colourless,
as what can take on sound is what is soundless; what is colourless
includes (a) what is transparent and (b) what is invisible or scarcely
visible, i.e. what is 'dark'. The latter (b) is the same as what is
transparent, when it is potentially, not of course when it is actually
transparent; it is the same substance which is now darkness, now
light.
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