195. Hume begins with an account of the ‘perceptions of the human mind,’ which corresponds to Locke’s account of ideas with two main qualifications, both tending to complete that dependence of thought on something other than itself which Locke had asserted, but not consistently maintained. He distinguishes ‘perceptions’ (equivalent to Locke’s ideas) into ‘impressions’ and ‘ideas’ accordingly as they are originally produced in feeling or reproduced by memory and imagination, and he does not allow ‘ideas of reflection’ any place in the _original_ ‘furniture of the mind.’ ‘An impression first strikes upon the senses, and makes us perceive heat or cold, thirst or hunger, pleasure or pain, of some kind or other. Of this impression there is a copy taken by the mind, which remains after the impression ceases; and this we call an idea. This idea of pleasure or pain, when it returns upon the soul, produces the new impressions of desire and aversion, hope and fear, which may properly be called impressions of reflection, because derived from it. These, again, are copied by the memory and imagination, and become ideas; which, perhaps, in their turn give rise to other impressions and ideas; so that the impressions are only antecedent to their correspondent ideas, but posterior to those of sensation and derived from them’ (Part I. §2). He is at the same time careful to explain that the causes from which the impressions of sensation arise are unknown (ibid.), and that by the term ‘impression’ he is not to be ‘understood to express the manner in which our lively perceptions are produced in the soul, but merely the perceptions themselves’. [1] The distinction between impression and idea he treats as equivalent to that between feeling and thinking, which, again, lies merely in the different degrees of ‘force and liveliness’ with which the perceptions, thus designated, severally ‘strike upon the mind.’ [2] Thus the rule which he emphasises [3] ‘that all our simple ideas in their first appearance are derived from simple impressions which are correspondent to them and which they exactly represent,’ strictly taken, means no more than that a feeling must be more lively before it becomes less so. As the reproduced perception, or ‘idea,’ differs in this respect from the original one, so, according to the greater or less degree of secondary liveliness which it possesses, is it called ‘idea of memory,’ or ‘idea of imagination.’ The only other distinction noticed is that, as might be expected, the comparative faintness of the ideas of imagination is accompanied by a possibility of their being reproduced in a different order from that in which the corresponding ideas were originally presented. Memory, on the contrary, ‘is in a manner tied down in this respect, without any power of variation’; [4] which must be understood to mean that, when the ideas are faint enough to allow of variation in the order of reproduction, they are not called ‘ideas of memory.’