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ing to which the passage from not being to being belongs； whether already actual； if its existence is due to a past process of becoming； or not yet actual but only possible。 The uses of the words 'destructible' and 'indestructible' are similar。 'Destructible' is applied （a） to that which formerly was and afterwards either is not or might not be； whether a period of being destroyed and changed intervenes or not； and （b） sometimes we apply the word to that which a process of destruction may cause not to be； and also （c） in a third sense； to that which is easily destructible； to the 'easily destroyed'； so to speak。 Of the indestructible the same account holds good。 It is either （a） that which now is and now is not； without any process of destruction； like contact； which without being destroyed afterwards is not； though formerly it was； or （b） that which is but might not be； or which will at some time not be； though it now is。 For you exist now and so does the contact； yet both are destructible； because a time will come when it will not be true of you that you exist； nor of these things that they are in contact。 Thirdly （c） in its most proper use； it is that which is； but is incapable of any destruction such that the thing which now is later ceases to be or might cease to be； or again； that which has not yet been destroyed； but in the future may cease to be。 For indestructible is also used of that which is destroyed with difficulty。 This being so； we must ask what we mean by 'possible' and 'impossible'。 For in its most proper use the predicate 'indestructible' is given because it is impossible that the thing should be destroyed； i。e。 exist at one time and not at another。 And 'ungenerated' also involves impossibility when used for that which cannot be generated； in such fashion that； while formerly it was not； later it is。 An instance is a commensurable diagonal。 Now when we speak of a power to move or to lift weights； we refer always to the maximum。 We speak； for instance； of a power to lift a hundred talents or walk a hundred stades…though a power to effect the maximum is also a power to effect any part of the maximum…since we feel obliged in defining the power to give the limit or maximum。 A thing； then； which is within it。 If； for example； a man can lift a hundred talents； he can also lift two； and if he can walk a hundred stades； he can also walk two。 But the power is of the maximum； and a thing said； with reference to its maximum； to be incapable of so much is also incapable of any greater amount。 It is； for instance； clear that a person who cannot walk a thousand stades will also be unable to walk a thousand and one。 This point need not trouble us； for we may take it as settled that what is； in the strict sense； possible is determined by a limiting maximum。 Now perhaps the objection might be raised that there is no necessity in this； since he who sees a stade need not see the smaller measures contained in it； while； on the contrary； he who can see a dot or hear a small sound will perceive what is greater。 This； however； does not touch our argument。 The maximum may be determined either in the power or in its object。 The application of this is plain。 Superior sight is sight of the smaller body； but superior speed is that of the greater body。
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Having established these distinctions we car now proceed to the sequel。 If there are thing！ capable both of being and of not being； there must be some definite maximum time of their being and not being； a time； I mean； during which continued existence is possible to them and a time during which continued nonexistence is possible。 And this is true in every category； whether the thing is； for example； 'man'； or 'white'； or 'three cubits long'； or whatever it may be。 For if the time is not definite in quantity； but longer than any that can be suggested and shorter than none； then it will be possible for one and the same thing to exist for infinite time and not to exist for another infinity。 This； however； is impossible。 Let us take our start from this point。 The impossible and the false have not the same significance。 One use of 'impossible' and 'possible'； and 'false' and 'true'； is hypothetical。 It is impossible； for instance； on a certain hypothesis that the triangle should have its angles equal to two right angles； and on another the diagonal is commensurable。 But there are also things possible and impossible； false and true； absolutely。 Now it is one thing to be absolutely false； and another thing to be absolutely impossible。 To say that you are standing when you are not standing is to assert a falsehood； but not an impossibility。 Similarly to say that a man who is playing the harp； but not singing； is singing； is to say what is false but not impossible。 To say； however； that you are at once standing and sitting； or that the diagonal is commensurable； is to say what is not only false but also impossible。 Thus it is not the same thing to make a false and to make an impossible hypothesis； and from the impossible hypothesis impossible results follow。 A man has； it is true； the capacity at once of sitting and of standing； because when he possesses the one he also possesses the other； but it does not follow that he can at once sit and stand； only that at another time he can do the other also。 But if a thing has for infinite time more than one capacity； another time is impossible and the times must coincide。 Thus if a thing which exists for infinite time is destructible； it will have the capacity of not being。 Now if it exists for infinite time let this capacity be actualized； and it will be in actuality at once existent and non…existent。 Thus a false conclusion would follow because a false assumption was made； but if what was assumed had not been impossible its consequence would not have been impossible。 Anything then which always exists is absolutely imperishable。 It is also ungenerated； since if it was generated it will have the power for some time of not being。 For as that which formerly was； but now is not； or is capable at some future time of not being； is destructible； so that which is capable of formerly not having been is generated。 But in the case of that which always is； there is no time for such a capacity of not being； whether the supposed time is finite or infinite； for its capacity of being must include the finite time since it covers infinite time。 It is therefore impossible that one and the same thing should be capable of always existing and of always not…existing。 And 'not always existing'； the contradictory； is also excluded。 Thus it is impossible for a thing always to exist and yet to be destructible。 Nor； similarly； can it be generated。 For of two attributes if B cannot be present without A； the impossibility A of proves the impossibility of B。 What always is； then； since it is incapable of ever not being； cannot possibly be generated。 But since the contradictory of 'that which is always capable of being' 'that which is not always capable of being'； while 'that which is always capable of not being' is the contrary； whose contradictory in turn is 'that which is not always capable of not being'； it is necessary that the contradi