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ensity of the stimulus must increase in the proportion that the intensity of the sensation is to increase； i。 e。； if a stimulus of 20 units requires the addition of 3 before it can be perceived； a stimulus of 60 units would require the addition of 9。 This law； which is of immense importance to criminalists who are discussing the sense…perceptions of witnesses； has been thoroughly and conclusively dealt with by A。 Meinong。＇1＇
＇1＇ Meinong： ber die Bedeutung der Weberschen Gesetzes。 Hamburg and Leipzig； 1896。
‘‘Modern psychology takes qualities perceived externally to be in themselves subjective but capable of receiving objectivity through our relation to the outer world。。。。 The qualitative character of our sensory content produced by external stimuli depends primarily on the organization of our senses。 This is the fundamental law of perception； of modern psychology； variously expressed； but axiomatic in all physiological psychology。''＇1＇ In this direction Helmholtz＇2＇ has done pioneer work。 He treats particularly the problem of optics； and physiological optics is the study of perception by means of the sense of sight。 We see things in the external world through the medium of light which they direct upon our eyes。 The light strikes the retina； and causes a sensation。 The sensation brought to the brain by means of the optic nerve becomes the condition of the representation in consciousness of certain objects distributed in space。。。。 We make use of the sensation which the light stimulates in the mechanism of the optic nerve to construct representations concerning the existence； form； and condition of external objects。 Hence we call images perceptions of sight。 （Our sense…perception； according to this theory； consists； therefore； entirely of sensations； the latter constitute the stuff or the content from which the other is constructed）。 Our sensations are effects caused in our organs； externally； and the manifestation of such an effect depends essentially upon the nature of the apparatus which has been stimulated。
＇1＇ T。 Pesch Das Weltphnomen
＇2＇ H。 Helmholtz： Die Tatsachen der Wahrnehmung。 Braunsehweig 1878。
There are certain really known inferences； e。 g。； those made by the astronomer from the perspective pictures of the stars to their positions in space。 These inferences are founded upon well… studied knowledge of the principles of optics。 Such knowledge of optics is lacking in the ordinary function of seeing； nevertheless it is permissible to conceive the psychical function of ordinary perception as unconscious inferences； inasmuch as this name will completely distinguish them from the commonly so…called conscious inferences。
The last…named condition is of especial importance to us。 We need investigation to determine the laws of the influence of optical and acoustical knowledge upon perception。 That these laws are influential may be verified easily。 Whoever is ignorant； e。 g。； that a noise is reflected back considerably； will say that a wagon is turning from the side from which the noise comes； though if he knows the law； if he knows that fact； his answer would be reversed。 So； as every child knows that the reflection of sound is frequently deceptive； everybody who is asked in court will say that he believes the wagon to be on the right side though it might as well have been on the left。 Again； if we were unaware that light is otherwise refracted in water than in air we could say that a stick in the water has been bent obtusely； but inasmuch as everybody knows this fact of the relation of light to water； he will declare that the stick appears bent but really is straight。
From these simplest of sense…perceptions to the most complicated； known only to half a dozen foremost physicists； there is an infinite series of laws controlling each stage of perception； and for each stage there is a group of men who know just so much and no more。 We have； therefore； to assume that their perceptions will vary with the number and manner of their accomplishments； and we may almost convince ourselves that each examinee who has to give evidence concerning his sense…perception should literally undergo examination to make clear his scholarly status and thereby the value of his testimony。 Of course； in practice this is not required。 First of all we judge approximately a man's nature and nurture and according to the impression he makes upon us； thence； his intellectual status。 This causes great mistakes。 But； on the other hand； the testimony is concerned almost always with one or several physical events； so that a simple relational interrogation will establish certainly whether the witness knows and attends to the physical law in question or not。 But anyway； too little is done to determine the means a man uses to reach a certain perception。 If instantaneous contradictions appear； there is little damage； for in the absence of anything certain； further inferences are fortunately made in rare cases only。 But when the observation is that of one person alone； or even when more testify but have accidentally the same amount of knowledge and hence have made the same mistake； and no contradiction appears； we suppose ourselves to possess the precise truth； confirmed by several witnesses； and we argue merrily on the basis of it。 In the meantime we quite forget that contradictions are our salvation from the trusting acceptance of untruth and that the absence of contradiction means； as a rule； the absence of a starting point for further examination。
For this reason and others modern psychology requires us to be cautious。 Among the others is the circumstance that perceptions are rarely pure。 Their purity consists in containing nothing else than perception； they are mixed when they are connected with imaginations； judgments； efforts； and volitions。 How rarely a perception is pure I have already tried to show； judgments almost always accompany it。 I repeat too； that owing to this circumstance and our ignorance of it； countless testimonies are interpreted altogether falsely。 This is true in many other fields。 When； for example； A。 Fick says： ‘‘The condition we call sensation occurs in the consciousness of the subject when his sensory nerves are stimulated；'' he does not mean that the nervous stimulus in itself is capable of causing the condition in question。 This one stimulus is only a single tone in the murmur of countless stimuli； which earlier and at the same time have influenced us and are different in their effect on each man。 Therefore； that single additional tone will also be different in each man。 Or； when Bernstein says that ‘‘Sensation； i。 e。； the stimulation of the sensorium and the passage of this stimulation to the brain； does not in itself imply the perception of an object or an event in the external world；'' we gather that the objectivity of the perception works correctively not more than one time out of many。 So here again everything depends upon the nature and nurture of the subject。
Sensations are； according to Aubert； still more subjective。 ‘‘They are the specific activity of the sense organs； （not； therefore； passive as according to Helmholtz； but active functions of the sense organs）。 P