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On The Soul   

The object of sight is the visible, and what is visible is (a)

colour and (b) a certain kind of object which can be described in

words but which has no single name; what we mean by (b) will be

abundantly clear as we proceed. Whatever is visible is colour and

colour is what lies upon what is in its own nature visible; 'in its

own nature' here means not that visibility is involved in the

definition of what thus underlies colour, but that that substratum

contains in itself the cause of visibility. Every colour has in it the

power to set in movement what is actually transparent; that power

constitutes its very nature. That is why it is not visible except with

the help of light; it is only in light that the colour of a thing is

seen. Hence our first task is to explain what light is.

Now there clearly is something which is transparent, and by

'transparent' I mean what is visible, and yet not visible in itself,

but rather owing its visibility to the colour of something else; of

this character are air, water, and many solid bodies. Neither air

nor water is transparent because it is air or water; they are

transparent because each of them has contained in it a certain

substance which is the same in both and is also found in the eternal

body which constitutes the uppermost shell of the physical Cosmos.

Of this substance light is the activity-the activity of what is

transparent so far forth as it has in it the determinate power of

becoming transparent; where this power is present, there is also the

potentiality of the contrary, viz. darkness. Light is as it were the

proper colour of what is transparent, and exists whenever the

potentially transparent is excited to actuality by the influence of

fire or something resembling 'the uppermost body'; for fire too

contains something which is one and the same with the substance in


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

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