384 OPERATIONS SUBSIDIARY TO INDUCTION. 



proprlate problem of logic, the estimation of evidence. We have to 

 consider, not~how or what to observe, but under what conditions ob- 

 servation is to be relied on; what is needful, in order that the fact, 

 supposed to be observed, may safely be received as true. 



§ 2. The answer to this question is very simple, at least in its first 

 aspect. The sole condition is, that what is supposed to have been 

 observed shall really have been observed ; that it be an observation, 

 not an inference. For in almost every act of our perceiving faculties, 

 observation and inference are intimately blended. What we are said 

 •o obsetve is usually a compound result, of which one-tenth may be 

 observation and the remaining nine-tenths inference. 



I affirm, for exariiple, that I hear a man's voice. This would pass, 

 in common language, for a direct perception. All, however, wl^ich is 

 really perception, is that I hear a sound. That the sound is a voice, 

 and that voice tiie voice of a rnan, are not perceptions but inferences. 

 I affirm, again, that I saw my brother at a certain hour this moming. 

 If any proposition concerning a matter of fact would commonly be 

 said to be knovsoi by the direct testimony of the senses, this surely 

 would be so. The truth, however^ is far otherwise. I only saw a 

 certain colored surface ; or rather I had the kind of visual sensations 

 which are usually produced by a colored surface ; and from these 

 as marks, known to be such by previous experience, I concluded that I 

 saw my brother. I might have had sensations precisely similar, when 

 my brother was not there. I might have seen some other person 

 so nearly resembling him in appearance, as, at the distance, and with 

 the degree of attention which I bestowed, to be mistaken for him. I 

 might have been asleep and have dreamed that I saw him ; or in a 

 state of nervous disorder, which brought his image before me in a 

 waking hallucination. In all these modes men have been led to be- 

 lieve that they saw persons well known to them, who were dead, or 

 far distant. If any of these suppositions had been true, the affirmation 

 that I saw my brother would have been eiToneous ; but whatever was 

 matter of direct perception, namely, the visual sensations, would have 

 been real. The inference only would have been ill grounded ; I 

 should have ascribed those sensations to a wrong cause. 



Innumerable instances might be given, and analyzed in the same 

 manner, of what are vulgarly, called errors of sense. There are none 

 of them properly eiTors of sense ; they are eiToneous inferences from 

 sense. When I look at a candle through a multiplying glass, 1- seem 

 to see a dozen candles instead of one: and if the real -circumstances of 

 the case were skillfully disguised, I might suppose that there were really 

 that number; there would be what is called an optical deception. In 

 the kaleidoscope there really is that deception : when I look through 

 the instrument, instead of what is actually there, namely, a casual ar- 

 rangement of colored fragments of glass, I seem to see the same com- 

 bination several times repeated in symmetrical arrangement round a 

 point. The delusioii is of course eftected by giving me the same sen- 

 sations, which I should have had if such a symmetrical combination had 

 really been presented to me. If I cross two of my fingers, and bring 

 any small object, a niarble for instance, into contact with both, at points 

 not usually touched simidtaneously by one object, I can hai'dly, if my 

 eyes are shut, help beheving that tiiere are two marbles instead of one. 



