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velopment of an existent quality from potentiality in the direction of fixity or nature。 In the case of what is to possess sense； the first transition is due to the action of the male parent and takes place before birth so that at birth the living thing is； in respect of sensation； at the stage which corresponds to the possession of knowledge。 Actual sensation corresponds to the stage of the exercise of knowledge。 But between the two cases compared there is a difference； the objects that excite the sensory powers to activity； the seen； the heard； &c。； are outside。 The ground of this difference is that what actual sensation apprehends is individuals； while what knowledge apprehends is universals； and these are in a sense within the soul。 That is why a man can exercise his knowledge when he wishes； but his sensation does not depend upon himself a sensible object must be there。 A similar statement must be made about our knowledge of what is sensible…on the same ground； viz。 that the sensible objects are individual and external。 A later more appropriate occasion may be found thoroughly to clear up all this。 At present it must be enough to recognize the distinctions already drawn； a thing may be said to be potential in either of two senses； （a） in the sense in which we might say of a boy that he may become a general or （b） in the sense in which we might say the same of an adult； and there are two corresponding senses of the term 'a potential sentient'。 There are no separate names for the two stages of potentiality； we have pointed out that they are different and how they are different。 We cannot help using the incorrect terms 'being acted upon or altered' of the two transitions involved。 As we have said； has the power of sensation is potentially like what the perceived object is actually； that is； while at the beginning of the process of its being acted upon the two interacting factors are dissimilar； at the end the one acted upon is assimilated to the other and is identical in quality with it。
6
In dealing with each of the senses we shall have first to speak of the objects which are perceptible by each。 The term 'object of sense' covers three kinds of objects； two kinds of which are； in our language； directly perceptible； while the remaining one is only incidentally perceptible。 Of the first two kinds one （a） consists of what is perceptible by a single sense； the other （b） of what is perceptible by any and all of the senses。 I call by the name of special object of this or that sense that which cannot be perceived by any other sense than that one and in respect of which no error is possible； in this sense colour is the special object of sight； sound of hearing； flavour of taste。 Touch； indeed； discriminates more than one set of different qualities。 Each sense has one kind of object which it discerns； and never errs in reporting that what is before it is colour or sound （though it may err as to what it is that is coloured or where that is； or what it is that is sounding or where that is。） Such objects are what we propose to call the special objects of this or that sense。 'Common sensibles' are movement； rest； number； figure； magnitude； these are not peculiar to any one sense； but are common to all。 There are at any rate certain kinds of movement which are perceptible both by touch and by sight。 We speak of an incidental object of sense where e。g。 the white object which we see is the son of Diares； here because 'being the son of Diares' is incidental to the directly visible white patch we speak of the son of Diares as being （incidentally） perceived or seen by us。 Because this is only incidentally an object of sense； it in no way as such affects the senses。 Of the two former kinds； both of which are in their own nature perceptible by sense； the first kind…that of special objects of the several senses…constitute the objects of sense in the strictest sense of the term and it is to them that in the nature of things the structure of each several sense is adapted。
7
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 question。 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。 Not everything that is visible depends upon light for its visibility。 This is only true of the 'proper' colour of things。 Some objects of sight which in light are invisible； in darkness stimulate the sense； that is； things that appear fiery or shining。 This class of objects has no simple common name； but instances of it are fungi； flesh； heads； scales； and eyes of fish。 In none of these is what is seen their own proper' colour。 Why we see these at all is another question。 At present what is obvious is that what is seen in light is always colour。 That is why without the help of light colour remains invisible。 Its being colour at all means precisely its having in it the power to set in movement what is already actually transparent； and； as we have seen； the actuality of what is transparent is just light。 The following experiment makes the necessity of a medium clear。 If what has colour is placed in immediate contact with the eye； it cannot be seen。 Colour sets in movement not the sense organ but what is transparent； e。g。 the air； and that； extending continuously from the object to the organ； sets the latter in movement。 Democritus misrepresents the facts when he expresses the opinion that if the interspace were empty one could distinctly see an ant on the vault of the sky； that is an impossibility。 Seeing is due to an affection or change of what has the perceptive faculty； and it cannot be affected by the seen colour itself； it remains that it must be affected by what comes between。 Hence it is indispensable that there be something in between…if there were nothing； so far from seeing with greater distinctness； we should see nothing at all。 We have now explained the cause why colour cannot be seen otherwise than in light。 Fire on the other hand is seen both in darkness and in light； this double possibility follows