‘a perception of the disagreement of men with one another, and an inquiry into the cause of this disagreement; a condemnation and distrust of that which only seems, and some kind of investigation of that which seems, as to whether it rightly seems: and the discovery of some rule (κανών)[13].’
Of all kinds of knowledge that which comes through the senses appears to the ordinary man most worthy of confidence, and of the five senses that of sight seems to the philosopher the most divine[14]. In consequence, the whole controversy hinges on the question whether the eyes can be trusted. The positivist argues that the evidence of sight is so plain and unmistakeable that man, if he had the choice, could wish for no better informant. The sceptic replies that nevertheless, if a straight oar be placed partly in the water, it appears to the eyes to be bent; and that the feathers on a dove’s neck, though really alike, appear to the eyes as many-coloured[15]. To deal with such questions we must examine closely the nature of sensation.
Process of sensation.
146. The Stoics fancifully derive the word αἴσθησις (‘sensation’) from εἴσθεσις (‘storage’); it is therefore, strictly speaking, the process by which the mind is stored[16]; but it is also, from an opposite point of view, the process by which the mind reaches out towards an external object[17]. From the object (αἰσθητόν) proceed waves which strike upon the sense-organ (αἰσθητήριον); this impact is called a ‘sensation’ in a narrower sense. At the same time there proceeds from the mind (which is the ruling part or ‘principate’ of the soul), a ‘spirit’ or thrill which goes out to meet this impact; and this spirit and its operation are also called ‘sensation[18].’ As a result of the contact of these two waves, and simultaneously with it, there is produced in the soul an effect like the imprint of a seal[19], and this imprint is the φαντασία or ‘mind-picture.’ That the process may be sound, it is necessary that the intellect be in a healthy state, and further that the organ of sense be healthy, the object really there, and the place and the manner in accord[20]. But we must carefully distinguish between the single sensation and the mind-picture. A flash of light, a cry, a touch, a smell, a thrill of pleasure or pain, is always that which the senses declare it to be[21]; here there is no possibility of error; so understood ‘the sensations are always true[22].’ But if we go in each case a step further; if we say ‘that is white,’ ‘this is sweet,’ ‘this is musical,’ ‘this is fragrant,’ ‘that is rough,’ we are now dealing with mind-pictures, not with ‘sensations’ in the strict sense[23]. And as to the mind-pictures we agree with the Academics that things are not always what they seem; ‘of the mind-pictures some are true, some are false[24].’
The criterion of clearness.
147. In order then that we may distinguish the true mind-picture from the false, we have need of a ‘rule’ (κανών) or ‘criterion’ (κριτήριον). The true mind-picture is a stirring of the soul, which reveals both what is taking place in the soul and the object which has caused this: just as light reveals both itself and the objects that lie within its range[25]. On the other hand the false mind-picture is an empty twitching of a soul which is not in a healthy condition[26]; no real object corresponds to it, but to that which appears to be an object corresponding to it we give the name ‘phantasm[27].’ When Orestes thinks he sees the Furies leaping upon him, though his sister assures him that in real truth he sees nothing, the vision of the Furies is a phantasm. The appearances of dreams are equally phantasms[28]. Now a true mind-picture differs from that of a phantasm by being clearer; or, in other words, the distinctive note of a true mind-picture is its ‘clearness’ (ἐνάργεια, perspicuitas)[29]. Clearness then is a quality which attaches itself to a true vision in a way in which it can never attach itself to a work of phantasy[30]. To this clearness the mind cannot but bow[31]; it is therefore (so far as our study has proceeded) the criterion of truth[32].
Assent.
148. The mind-picture as such is not within a man’s control; but it rests with him to decide whether he will give it his ‘assent’ (συγκατάθεσις, adsensio or adsensus)[33]. This assent is therefore an act of the soul, in its capacity as will; and can only be rightly exercised by a soul properly strung, that is, possessed of due tension. Assent wrongly given leads to ‘opinion’ (δόξα, opinio), and all wrong assent is error or ‘sin’ (ἁμαρτία, peccatum). This error may take place in two directions, either by a hasty movement of the will (προπίπτειν), giving assent to a picture which is not really clear; or by feebleness of will, which leads to assent in a false direction (διαψεύδεσθαι)[34]. Even haste however is a form of weakness, so that we may say that all opining is a weak form of assent[35]. To ensure a right assent due attention should be given to each of its parts; it includes (i) the intention of mastering the object (πρόθεσις); (ii) careful attention directed to the object, or ‘application’ (ἐπιβολή); and (iii) assent in the narrower sense[36]. Apart from assent, three courses remain open: these are (i) ‘quiescence’ (ἡσυχάζειν, quiescere): (ii) ‘suspense of judgment’ (ἐπέχειν, adsensum sustinere), which is a settled quiescence; and (iii) negation[37].
Comprehension.
149. Close upon assent follows ‘comprehension’ (κατάληψις, comprehensio): this is the ratification of the assent given, the fixing irrevocably in the mind of the picture approved. This picture now becomes a ‘comprehension-picture’ (καταληπτικὴ φαντασία), and as such a unit of knowledge. We may understand thereby that the mind has grasped the external object[38], and this is the plain meaning of Zeno’s simile; or we may say that the object has gained a hold upon the mind, and has left its stamp upon it. Both interpretations are consistent with Stoic doctrine: but the former view, which represents the soul as active and masterful, undoubtedly expresses the more adequately the meaning of the school[39]. From this mutual grasp there follows an important physical deduction. Since only like can grasp like, the soul must be like the object, and the popular dualism of mind and matter is (to this extent) at an end[40]. Still this likeness is not complete; and the soul in sensation does not grasp the object from every point of view, but only so far as its own nature permits in each case[41]. For this reason the trained observer and the artist grasp far more of the object than the ordinary man[42].