I am going to quote one little section in a book called Hume’s First Principles by Robert Fendel Anderson. This first part of the book is on Perceptions, and the first principle gone over on Perceptions is “Whatever is Conceivable is Possible”.
“The principle of the possible existence of whatever is conceivable is one which Hume finds both an evident principle and already an established maxim in metaphysics[1]. The application of the principle is frequently restricted to that which is clearly and distinctly conceivable: “…nothing of which we can form a clear and distinct idea is absurd and impossible.”[2] Again: “To form a clear idea of anything, is an undeniable argument for its possibility…”[3]. The possibility of existence, therefore, is of the essence of whatever is clearly and distinctly conceived; that is, its possibility is included or implied within it: “ ‘Tis an establish’d maxim in metaphysics, That whatever the mind clearly conceives includes the idea of possible existence…”[4] and: “Whatever can be conceiv’d by a clear and distinct idea necessarily implies the possibility of existence….”[5]
A clear and distinct idea, according to Hume’s doctrine, is one which neither contains nor implies a contradiction: “Now whatever is intelligible, and can be distinctly conceived, implies no contradiction…”[6] Again: “How any clear, and distinct idea can contain circumstances, contradictory to itself, or to any other clear, distinct idea, is absolutely incomprehensible….”[7] In saying that whatever is clearly and distinctly conceived is possible, therefore, it appears to be Hume’s intention also that whatever is self-consistent and noncontradictory is possible:
“Whatever can be conceiv’d by a clear and distinct idea necessarily implies the possibility of existence; and he who pretends to prove the impossibility of its existence by any argument deriv’d from the clear idea, in reality asserts, that we have no clear idea of it, because we have a clear idea. ‘Tis in vain to search for a contradiction in any thing that is distinctly conceiv’d by the mind.”[8]
The expression employed in the remarks thus far examined may lead the reader to suppose that there are some things clearly and distinctly conceived and some not- that some of our ideas are clear and distinct and some of them unclear and indistinct. Were this true, then it would follow that we have ideas of things the existence of which we must regard as impossible. There is evidence, however, that Hume considers all our ideas to be clear and distinct. He offers an argument to this conclusion, based on his doctrine that ideas are derived from impressions:
“…we need but reflect on that principle so oft insisted on, that all our ideas are copy’d from our impressions. For from thence we may immediately conclude, that since all impressions are clear and precise, the ideas, which are copy’d from them, must be of the same nature…”[9]
Since all perceptions are either impressions or ideas[10], we must conclude that there are no perceptions of any kind that are not clear and precise.
From the clarity and preciseness of all ideas, we may infer, moreover, that we possess no ideas of those things whose existence we must regard as impossible, but that any idea we may have is the idea of something the existence of which is possible. We find, indeed, that Hume does not always restrict the possibility of existence to that which is clearly and distinctly conceived, but extends it as well to everything that is conceived or imagined at all: “…whatever we conceive is possible.”[11] And: “…whatever we can imagine, is possible.”[12]Hume appears, indeed, to make no firm distinction between what is clearly and distinctly conceived and what is conceived or imagined merely, as is evidenced in his full statement of the metaphysical maxim: “ ‘Tis an establish’d maxim in metaphysics, That whatever the mind clearly conceives includes the idea of possible existence, or in other words, that nothing we imagine is absolutely impossible.”[13] We are thus again justified, apparently, in supposing that all our ideas are equally clear and distinct, and that all things conceived are possible. Things which are contradictory and therefore impossible, on the other hand, cannot be conceived or imagined at all: “We can form the idea of a golden mountain, and from thence conclude that such a mountain may actually exist. We can form no idea of a mountain without a valley, and therefore regard it as impossible.”[14] Again: “ ‘Tis in vain to search for a contradiction in any thing that is distinctly conceiv’d by the mind. Did it imply any contradiction, ‘tis impossible it cou’d ever be coneiv’d.”[15]
Knowing then that self-contradictory things are neither conceivable nor possible, and knowing that whatever is conceived or imagined is possible, we may next inquire what things are in fact conceived or imagined and hence possible. From certain of Hume’s remarks one might infer that we conceive only perceptions; for it is only perceptions that are “present to” the mind: “…nothing is ever really present with the mind but its perceptions or impressions and ideas…”[16] If this be true, then it is reasonable to suppose that we have clear and distinct ideas only of perceptions, as Hume sometimes appears to agree: “We have no perfect idea of any thing but of a perception.”[17] Now if we can conceive only of perceptions, then according to Hume’s principle it is only perceptions whose existence we may regard as possible. We may observe, moreover, that the remarks we have thus far examined do not imply that perceptions, as such, exist, but only that their existence is possible. Were there no further texts available to us from among Hume’s writings, we might justifiably conclude that what he calls “perceptions” are to be understood as a realm of mere essences which, taken together, comprehend all possibility, but which are not, of themselves, existence.”
[1] David Hume, A Treatise of Human Nature, ed. by L.A. Selby-Bigge (Oxford: Clarendon Press, 1888), pp. 32, 250, Hereafter cited as Treatise.
[2] Treatise, pp.19-20. Cf> David Hume, Dialogues Concerning Natural Religion, ed. and with an introduction by Henry D. Aiken(New York: Hafner Library of Classic, Hafner Publishing Company, 1948), p. 19, Philo speaking. Hereafter cited as Dialogues.
[3] Treatise, p. 89
[4] Treatise, p.32
[5] Treatise, p. 43
[6] David Hume, “An Enquiry Concerning the Human Understanding,” in An Enquiry Concerning the Human Understanding and an Enquiry Concerning the Principles of Morals, ed. and with an introduction by L.A. Selby-Bigge (2d ed.; Oxford: Clarendon Press, 1902), p. 35. Hereafter cited as “Understanding.” Cf. Dialogues, p. 58, Cleanthes speaking.
[7] “Understanding.” P. 157
[8] Treatise p. 43.
[9] Treatise, p. 72; cf. p. 366.
[10] Treatise, pp. 1, 96.
[11] Treatise, p. 236.
[12] Treatise, p. 250
[13] Treatise, p. 32.
[14] Treatise, p. 32.
[15] Treatise, p.43. Cf. “Understanding,” p. 164.
[16] Treatise, p. 67; cf. pp.197,212.
[17] Treatise, p. 234.
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