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at once white and black； and therefore it must also be impossible for a thing to be affected at one and the same moment by the forms of both； assuming it to be the case that sensation and thinking are properly so described。 The answer is that just as what is called a 'point' is； as being at once one and two； properly said to be divisible； so here； that which discriminates is qua undivided one； and active in a single moment of time； while so far forth as it is divisible it twice over uses the same dot at one and the same time。 So far forth then as it takes the limit as two' it discriminates two separate objects with what in a sense is divided： while so far as it takes it as one； it does so with what is one and occupies in its activity a single moment of time。 About the principle in virtue of which we say that animals are percipient； let this discussion suffice。
3
There are two distinctive peculiarities by reference to which we characterize the soul （1） local movement and （2） thinking； discriminating； and perceiving。 Thinking both speculative and practical is regarded as akin to a form of perceiving； for in the one as well as the other the soul discriminates and is cognizant of something which is。 Indeed the ancients go so far as to identify thinking and perceiving； e。g。 Empedocles says 'For 'tis in respect of what is present that man's wit is increased'； and again 'Whence it befalls them from time to time to think diverse thoughts'； and Homer's phrase 'For suchlike is man's mind' means the same。 They all look upon thinking as a bodily process like perceiving； and hold that like is known as well as perceived by like； as I explained at the beginning of our discussion。 Yet they ought at the same time to have accounted for error also； for it is more intimately connected with animal existence and the soul continues longer in the state of error than in that of truth。 They cannot escape the dilemma： either （1） whatever seems is true （and there are some who accept this） or （2） error is contact with the unlike； for that is the opposite of the knowing of like by like。 But it is a received principle that error as well as knowledge in respect to contraries is one and the same。 That perceiving and practical thinking are not identical is therefore obvious； for the former is universal in the animal world； the latter is found in only a small division of it。 Further； speculative thinking is also distinct from perceiving…I mean that in which we find rightness and wrongness…rightness in prudence； knowledge； true opinion； wrongness in their opposites； for perception of the special objects of sense is always free from error； and is found in all animals； while it is possible to think falsely as well as truly； and thought is found only where there is discourse of reason as well as sensibility。 For imagination is different from either perceiving or discursive thinking； though it is not found without sensation； or judgement without it。 That this activity is not the same kind of thinking as judgement is obvious。 For imagining lies within our own power whenever we wish （e。g。 we can call up a picture； as in the practice of mnemonics by the use of mental images）； but in forming opinions we are not free： we cannot escape the alternative of falsehood or truth。 Further； when we think something to be fearful or threatening； emotion is immediately produced； and so too with what is encouraging； but when we merely imagine we remain as unaffected as persons who are looking at a painting of some dreadful or encouraging scene。 Again within the field of judgement itself we find varieties； knowledge； opinion； prudence； and their opposites； of the differences between these I must speak elsewhere。 Thinking is different from perceiving and is held to be in part imagination； in part judgement： we must therefore first mark off the sphere of imagination and then speak of judgement。 If then imagination is that in virtue of which an image arises for us； excluding metaphorical uses of the term； is it a single faculty or disposition relative to images； in virtue of which we discriminate and are either in error or not？ The faculties in virtue of which we do this are sense； opinion； science； intelligence。 That imagination is not sense is clear from the following considerations： Sense is either a faculty or an activity； e。g。 sight or seeing： imagination takes place in the absence of both； as e。g。 in dreams。 （Again； sense is always present； imagination not。 If actual imagination and actual sensation were the same； imagination would be found in all the brutes： this is held not to be the case； e。g。 it is not found in ants or bees or grubs。 （Again； sensations are always true； imaginations are for the most part false。 （Once more； even in ordinary speech； we do not； when sense functions precisely with regard to its object； say that we imagine it to be a man； but rather when there is some failure of accuracy in its exercise。 And as we were saying before； visions appear to us even when our eyes are shut。 Neither is imagination any of the things that are never in error： e。g。 knowledge or intelligence； for imagination may be false。 It remains therefore to see if it is opinion； for opinion may be either true or false。 But opinion involves belief （for without belief in what we opine we cannot have an opinion）； and in the brutes though we often find imagination we never find belief。 Further； every opinion is accompanied by belief； belief by conviction； and conviction by discourse of reason： while there are some of the brutes in which we find imagination； without discourse of reason。 It is clear then that imagination cannot； again； be （1） opinion plus sensation； or （2） opinion mediated by sensation； or （3） a blend of opinion and sensation； this is impossible both for these reasons and because the content of the supposed opinion cannot be different from that of the sensation （I mean that imagination must be the blending of the perception of white with the opinion that it is white： it could scarcely be a blend of the opinion that it is good with the perception that it is white）： to imagine is therefore （on this view） identical with the thinking of exactly the same as what one in the strictest sense perceives。 But what we imagine is sometimes false though our contemporaneous judgement about it is true； e。g。 we imagine the sun to be a foot in diameter though we are convinced that it is larger than the inhabited part of the earth； and the following dilemma presents itself。 Either （a while the fact has not changed and the （observer has neither forgotten nor lost belief in the true opinion which he had； that opinion has disappeared； or （b） if he retains it then his opinion is at once true and false。 A true opinion； however； becomes false only when the fact alters without being noticed。 Imagination is therefore neither any one of the states enumerated； nor compounded out of them。 But since when one thing has been set in motion another thing may be moved by it； and imagination is held to be a movement and to be impossible without sensation； i。e。 to occur in beings that are percipient and to have for its content what can be perceived； and since mov