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occur in beings that are percipient and to have for its content what can be perceived； and since movement may be produced by actual sensation and that movement is necessarily similar in character to the sensation itself； this movement must be （1） necessarily （a） incapable of existing apart from sensation； （b） incapable of existing except when we perceive； （such that in virtue of its possession that in which it is found may present various phenomena both active and passive； and （such that it may be either true or false。 The reason of the last characteristic is as follows。 Perception （1） of the special objects of sense is never in error or admits the least possible amount of falsehood。 （2） That of the concomitance of the objects concomitant with the sensible qualities comes next： in this case certainly we may be deceived； for while the perception that there is white before us cannot be false； the perception that what is white is this or that may be false。 （3） Third comes the perception of the universal attributes which accompany the concomitant objects to which the special sensibles attach （I mean e。g。 of movement and magnitude）； it is in respect of these that the greatest amount of sense…illusion is possible。 The motion which is due to the activity of sense in these three modes of its exercise will differ from the activity of sense； （1） the first kind of derived motion is free from error while the sensation is present； （2） and （3） the others may be erroneous whether it is present or absent； especially when the object of perception is far off。 If then imagination presents no other features than those enumerated and is what we have described； then imagination must be a movement resulting from an actual exercise of a power of sense。 As sight is the most highly developed sense； the name Phantasia （imagination） has been formed from Phaos （light） because it is not possible to see without light。 And because imaginations remain in the organs of sense and resemble sensations； animals in their actions are largely guided by them； some （i。e。 the brutes） because of the non…existence in them of mind； others （i。e。 men） because of the temporary eclipse in them of mind by feeling or disease or sleep。 About imagination； what it is and why it exists； let so much suffice。
4
Turning now to the part of the soul with which the soul knows and thinks （whether this is separable from the others in definition only； or spatially as well） we have to inquire （1） what differentiates this part； and （2） how thinking can take place。 If thinking is like perceiving； it must be either a process in which the soul is acted upon by what is capable of being thought； or a process different from but analogous to that。 The thinking part of the soul must therefore be； while impassible； capable of receiving the form of an object； that is； must be potentially identical in character with its object without being the object。 Mind must be related to what is thinkable； as sense is to what is sensible。 Therefore； since everything is a possible object of thought； mind in order； as Anaxagoras says； to dominate； that is； to know； must be pure from all admixture； for the co…presence of what is alien to its nature is a hindrance and a block： it follows that it too； like the sensitive part； can have no nature of its own； other than that of having a certain capacity。 Thus that in the soul which is called mind （by mind I mean that whereby the soul thinks and judges） is； before it thinks； not actually any real thing。 For this reason it cannot reasonably be regarded as blended with the body： if so； it would acquire some quality； e。g。 warmth or cold； or even have an organ like the sensitive faculty： as it is； it has none。 It was a good idea to call the soul 'the place of forms'； though （1） this description holds only of the intellective soul； and （2） even this is the forms only potentially； not actually。 Observation of the sense…organs and their employment reveals a distinction between the impassibility of the sensitive and that of the intellective faculty。 After strong stimulation of a sense we are less able to exercise it than before； as e。g。 in the case of a loud sound we cannot hear easily immediately after； or in the case of a bright colour or a powerful odour we cannot see or smell； but in the case of mind thought about an object that is highly intelligible renders it more and not less able afterwards to think objects that are less intelligible： the reason is that while the faculty of sensation is dependent upon the body； mind is separable from it。 Once the mind has become each set of its possible objects； as a man of science has； when this phrase is used of one who is actually a man of science （this happens when he is now able to exercise the power on his own initiative）； its condition is still one of potentiality； but in a different sense from the potentiality which preceded the acquisition of knowledge by learning or discovery： the mind too is then able to think itself。 Since we can distinguish between a spatial magnitude and what it is to be such； and between water and what it is to be water； and so in many other cases （though not in all； for in certain cases the thing and its form are identical）； flesh and what it is to be flesh are discriminated either by different faculties； or by the same faculty in two different states： for flesh necessarily involves matter and is like what is snub…nosed； a this in a this。 Now it is by means of the sensitive faculty that we discriminate the hot and the cold； i。e。 the factors which combined in a certain ratio constitute flesh： the essential character of flesh is apprehended by something different either wholly separate from the sensitive faculty or related to it as a bent line to the same line when it has been straightened out。 Again in the case of abstract objects what is straight is analogous to what is snub…nosed； for it necessarily implies a continuum as its matter： its constitutive essence is different； if we may distinguish between straightness and what is straight： let us take it to be two…ness。 It must be apprehended； therefore； by a different power or by the same power in a different state。 To sum up； in so far as the realities it knows are capable of being separated from their matter； so it is also with the powers of mind。 The problem might be suggested： if thinking is a passive affection； then if mind is simple and impassible and has nothing in common with anything else； as Anaxagoras says； how can it come to think at all？ For interaction between two factors is held to require a precedent community of nature between the factors。 Again it might be asked； is mind a possible object of thought to itself？ For if mind is thinkable per se and what is thinkable is in kind one and the same； then either （a） mind will belong to everything； or （b） mind will contain some element common to it with all other realities which makes them all thinkable。 （1） Have not we already disposed of the difficulty about interaction involving a common element； when we said that mind is in a sense potentially whatever is thinkable； though actually it is nothing until it has thought？ What it thinks must