282. In the account just adduced of vacuum, the effect of that double dealing with ‘impressions,’ which we shall have to trace at large in Hume’s explanation of our language about Causation and Identity, is already exhibited in little. Just as, after the idea of pure space has been excluded because not a copy of any possible impression, we yet find an ‘idea,’ only differing from it in name, introduced as the basis of that tendency to suppose which is to take the place of the excluded idea, so we shall find ideas of relation in the way of Identity and Causation--ideas which according to Hume we have not--presupposed as the source of those ‘propensities to feign’ which he accounts for the appearance of our having them. Knowledge of relation in way of Identity and Causation excluded by Locke’s definition of knowledge. 283. The primary characteristic of these relations according to Hume, which they share with those of space and time, and which in fact vitiates that definition of ‘philosophical relation,’ as depending on comparison, which he adopts, is that they ‘depend not on the ideas compared together, but may be changed without any change in the ideas’. [1] It follows that they are not objects of knowledge, according to the definition of knowledge which Hume inherited, as ‘the perception of agreement or disagreement between ideas.’ A partial recognition of this consequence in regard to cause and effect we found in Locke’s suspicion that a science of nature was impossible--impossible because, however often a certain ‘idea of quality and substance’ may have followed or accompanied another, such sequence or accompaniment never amounts to agreement or ‘necessary connexion’ between the ideas, and therefore never can warrant a general assertion, but only the particular one, that the ideas in question have so many times occurred in such an order. ‘Matters of fact,’ however, which no more consist in agreement of ideas than does causation, are by Locke treated without scruple as matter of knowledge when they can be regarded as relations between present sensations. Thus the ‘particular experiment’ in Physics constitutes knowledge--the knowledge, for instance, that a piece of gold is now dissolved in aqua regia; and when ‘I myself see a man walk on the ice, it is knowledge.’ In such cases it does not occur to him to ask, either what are the ideas that agree or how much of the experiment is a present sensation. [2] Nor does Hume commonly carry his analysis further. After admitting that the relations called ‘identity and situation in time and place’ do not depend on the nature of the ideas related, he proceeds: ‘When both the objects are present to the senses along with the relation, we call _this_ perception rather than reasoning; nor is there in this case any exercise of the thought or any action, properly speaking, but a mere passive admission of the impressions through the organs of sensation. According to this