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ON THE NATURE OF THE NOTION OF EXTERNALITY. 209 show that he has imaginatively, but ' unconsciously,' sup- plied many links missing from actual perception. The facility with which observation is unconsciously supple- mented by inference is, indeed, a common-place of logic, and is a familiar fact to all who have had to sift evidence. At the same time, as the complement of this, we have the fact that we habitually neglect in any observation what we consider irrelevant concomitant sensations, which are re- garded as belonging to a different series of events ; as when in a chemical experiment we neglect the noises in the street. Both the neglect of ' irrelevant ' perceptions and the imaginative supplying of what is missing in perception are, however, most clearly marked where the ' irrelevant ' mental phenomena are not concomitant with our observa- tions, but intervene between different stages of these. When an astronomer, for example, resumes his observations of a planet which he has been studying the night before, he neglects the intervening perceptions and psychical states in general. He does not seek to tack his present perception of the planet on to these mental phenomena, which in point of time led up to it; but he connects his present obser- vations with those of last night, by imaginatively recon- structing the progressive shifting of position, as between the planet and the earth, which has proceeded independ- ently of his perceptions, thoughts or emotions. It is only from this point of view, it may be remarked in passing, that the doctrine of the continuity of the path of motion is anything but ridiculously false. It is, I fancy, needless to multiply examples in support of my contention, and, indeed, I feel that an apology is due to the reader for what may seem the absurd length at which I have dwelt upon the discontinuity in percep- tions. My excuse must be that this important truth has been too long neglected. Ueberweg is almost the only considerable philosopher who has insisted on the fact that perceptions do not succeed one another in a regular order. 1 The result of this neglect is seen in the possibility of such productions as the Grammar of Science. We may diagrammatically represent our results by the 1 In the notes appended to his translation of Berkeley's Principles of Human Knowledge under the title of Berkeley's Abhandlungen uber die Principinn d<r menschlichen Erkenntniss (second edition, 1879) : see especially Notes 45, 56 and 77. 14