175. This, it may seem, however inconsistent with the doctrine of primary qualities, is little more than the result which Locke himself comes to in his Fourth Book; since, if ‘actual present succession’ forms our only knowledge of real existence, there could be no time at which distance and body might be _known_ as really existing. But Locke, as we have seen, is able to save mathematical, though not physical, knowledge from the consequences of this admission by his doctrine of abstract ideas--‘ideas removed in our thoughts from particular existence’--whose agreement or disagreement is stated in propositions which ‘concern not existence,’ and for that reason may be general without becoming either uncertain or uninstructive. This doctrine Berkeley expressly rejects on the ground that he could not perceive separately that which could not exist separately (‘Principles of Human Knowledge,’ Introduction, sec. 10); a ground which to the ordinary reader seems satisfactory because he has no doubt, and Berkeley’s instances do not suggest a doubt, as to the present existence of ‘individual objects’--this man, this horse, this body. But with Berkeley to exist means to be felt (‘Principles of Human Knowledge,’ sec. 3), and the feelings, which I name a body, being successive, its existence must be in succession likewise. The limitation, then, of possibility of ‘conception’ by possibility of existence, means that ‘conception,’ too, is reduced to a succession of feelings. On the same principle all permanent relations should disappear. 176. Berkeley, then, as a consequence of the methods by which he disposes at once of the ‘real existence’ and ‘abstract idea of matter,’ has to meet the following questions:--How are either reality or knowledge possible without permanent relations? and, How can feelings, of which one is over before the next begins, constitute or represent a world of permanent relations? The difficulty becomes more obvious, though not more serious, when the relations in question are not merely themselves permanent, as are those between natural phenomena, but are relations between permanent parts like those of space. It is for this reason that its doctrine of geometry is the most easily assailable point of the ‘sensational’ philosophy. Locke distinguishes the ideas of space and of duration as got, the one from the permanent parts of space, the other ‘from the fleeting and perpetually perishing parts of succession.’ [1] He afterwards prefers to oppose the term ‘expansion’ to ‘duration,’ as bringing out more clearly than ‘space’ the opposition of relation between permanent facts to that between ‘fleeting successive facts which never exist together.’ How, then, can a consciousness, consisting simply of ‘fleeting successive facts,’ either be or represent that of which the differentia is that its facts are permanent and co-exist? [1] Book II. chap. xiv. sec. 1. By making colour = relations of coloured points, Berkeley represents relation as seen.