184 OPERATIONS SUBSIDIARY TO INDUCTION. 



consider, not how or what to observe, but under what con 

 ditions observation 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 inti 

 mately blended. What we are said to observe is usually a 

 compound result, of which one-tenth may be observation, and 

 the remaining nine-tenths inference. 



I affirm, for example, that I hear a man s voice. This 

 would pass, in common language, for a direct perception. All, 

 however, which is really perception, is that I hear a sound. 

 That the sound is a voice, and that voice the voice of a man, 

 are not perceptions but inferences. I affirm, again, that I saw 

 my brother at a certain hour this morning. If any proposition 

 concerning a matter of fact would commonly be said to be 

 known by the direct testimony of the senses, this surely would 

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

 certain coloured surface ; or rather I had the kind of visual 

 sensations which are usually produced by a coloured surface ; 

 and from these as marks, known to be such by previous expe 

 rience, 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, many have 

 been led to believe 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 

 erroneous ; but whatever was matter of direct perception, namely 

 the visual sensations, would have been real. The inference only 



