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rse of nature there is a limit or ratio which determines their size and increase； and limit and ratio are marks of soul but not of fire； and belong to the side of formulable essence rather than that of matter。 Nutrition and reproduction are due to one and the same psychic power。 It is necessary first to give precision to our account of food； for it is by this function of absorbing food that this psychic power is distinguished from all the others。 The current view is that what serves as food to a living thing is what is contrary to it…not that in every pair of contraries each is food to the other： to be food a contrary must not only be transformable into the other and vice versa； it must also in so doing increase the bulk of the other。 Many a contrary is transformed into its other and vice versa； where neither is even a quantum and so cannot increase in bulk； e。g。 an invalid into a healthy subject。 It is clear that not even those contraries which satisfy both the conditions mentioned above are food to one another in precisely the same sense； water may be said to feed fire； but not fire water。 Where the members of the pair are elementary bodies only one of the contraries； it would appear； can be said to feed the other。 But there is a difficulty here。 One set of thinkers assert that like fed； as well as increased in amount； by like。 Another set； as we have said； maintain the very reverse； viz。 that what feeds and what is fed are contrary to one another； like； they argue； is incapable of being affected by like； but food is changed in the process of digestion； and change is always to what is opposite or to what is intermediate。 Further； food is acted upon by what is nourished by it； not the other way round； as timber is worked by a carpenter and not conversely； there is a change in the carpenter but it is merely a change from not…working to working。 In answering this problem it makes all the difference whether we mean by 'the food' the 'finished' or the 'raw' product。 If we use the word food of both； viz。 of the completely undigested and the completely digested matter； we can justify both the rival accounts of it； taking food in the sense of undigested matter； it is the contrary of what is fed by it； taking it as digested it is like what is fed by it。 Consequently it is clear that in a certain sense we may say that both parties are right； both wrong。 Since nothing except what is alive can be fed； what is fed is the besouled body and just because it has soul in it。 Hence food is essentially related to what has soul in it。 Food has a power which is other than the power to increase the bulk of what is fed by it； so far forth as what has soul in it is a quantum； food may increase its quantity； but it is only so far as what has soul in it is a 'this…somewhat' or substance that food acts as food； in that case it maintains the being of what is fed； and that continues to be what it is so long as the process of nutrition continues。 Further； it is the agent in generation； i。e。 not the generation of the individual fed but the reproduction of another like it； the substance of the individual fed is already in existence； the existence of no substance is a self…generation but only a self…maintenance。 Hence the psychic power which we are now studying may be described as that which tends to maintain whatever has this power in it of continuing such as it was； and food helps it to do its work。 That is why； if deprived of food； it must cease to be。 The process of nutrition involves three factors； （a） what is fed； （b） that wherewith it is fed； （c） what does the feeding； of these （c） is the first soul； （a） the body which has that soul in it； （b） the food。 But since it is right to call things after the ends they realize； and the end of this soul is to generate another being like that in which it is； the first soul ought to be named the reproductive soul。 The expression （b） 'wherewith it is fed' is ambiguous just as is the expression 'wherewith the ship is steered'； that may mean either （i） the hand or （ii） the rudder； i。e。 either （i） what is moved and sets in movement； or （ii） what is merely moved。 We can apply this analogy here if we recall that all food must be capable of being digested； and that what produces digestion is warmth； that is why everything that has soul in it possesses warmth。 We have now given an outline account of the nature of food； further details must be given in the appropriate place。
5
Having made these distinctions let us now speak of sensation in the widest sense。 Sensation depends； as we have said； on a process of movement or affection from without； for it is held to be some sort of change of quality。 Now some thinkers assert that like is affected only by like； in what sense this is possible and in what sense impossible； we have explained in our general discussion of acting and being acted upon。 Here arises a problem： why do we not perceive the senses themselves as well as the external objects of sense； or why without the stimulation of external objects do they not produce sensation； seeing that they contain in themselves fire； earth； and all the other elements； which are the direct or indirect objects is so of sense？ It is clear that what is sensitive is only potentially； not actually。 The power of sense is parallel to what is combustible； for that never ignites itself spontaneously； but requires an agent which has the power of starting ignition； otherwise it could have set itself on fire； and would not have needed actual fire to set it ablaze。 In reply we must recall that we use the word 'perceive' in two ways； for we say （a） that what has the power to hear or see； 'sees' or 'hears'； even though it is at the moment asleep； and also （b） that what is actually seeing or hearing； 'sees' or 'hears'。 Hence 'sense' too must have two meanings； sense potential； and sense actual。 Similarly 'to be a sentient' means either （a） to have a certain power or （b） to manifest a certain activity。 To begin with； for a time； let us speak as if there were no difference between （i） being moved or affected； and （ii） being active； for movement is a kind of activity…an imperfect kind； as has elsewhere been explained。 Everything that is acted upon or moved is acted upon by an agent which is actually at work。 Hence it is that in one sense； as has already been stated； what acts and what is acted upon are like； in another unlike； i。e。 prior to and during the change the two factors are unlike； after it like。 But we must now distinguish not only between what is potential and what is actual but also different senses in which things can be said to be potential or actual； up to now we have been speaking as if each of these phrases had only one sense。 We can speak of something as 'a knower' either （a） as when we say that man is a knower； meaning that man falls within the class of beings that know or have knowledge； or （b） as when we are speaking of a man who possesses a knowledge of grammar； each of these is so called as having in him a certain potentiality； but there is a difference between their respective potentialities； the one （a） being a potential knower； because his kind or matter is such and such； the other （b）； because he can in the absence of any external counteracting cause realize his knowledge in actual knowing at will。 This implies a third meaning of 'a knower' （c）； one who is already realizing his knowledge…he is a knower in actuality and in the most proper sense is knowing； e。g。 this A。 Both the former are potential knowers； who realize their respective potentialities； the one （a） by change of quality； i。e。 repeated transitions from one state to its opposite under instruction； the other （b） by the transition from the inactive possession of sense or grammar to their active exercise。 The two kinds of transition are distinct。 Also the expression 'to be acted upon' has more than one meaning； it may mean either （a） the extinction of one of two contraries by the other； or （b） the maintenance of what is potential by the agency of what is actual and already like what is acted upon； with such likeness as is compatible with one's being actual and the other potential。 For what possesses knowledge becomes an actual knower by a transition which is either not an alteration of it at all （being in reality a development into its true self or actuality） or at least an alteration in a quite different sense from the usual meaning。 Hence it is wrong to speak of a wise man as being 'altered' when he uses his wisdom； just as it would be absurd to speak of a builder as being altered when he is using his skill in building a house。 What in the case of knowing or understanding leads from potentiality to actuality ought not to be called teaching but something else。 That which starting with the power to know learns or acquires knowledge through the agency of one who actually knows and has the power of teaching either （a） ought not to be said 'to be acted upon' at all or （b） we must recognize two senses of alteration； viz。 （i） the substitution of one quality for another； the first being the contrary of the second； or （ii） the development of an existent quality from potentiality in the direction of fixity or nature。 In the ca