Here, mostly for my own benefit, are the major passages on feigning and fiction in Hume's Treatise; it shows the importance of it for some of Hume's key theses. (It does not include every passage; there are quite a few mentions of fictions in 1.3.10, for instance, but the word is usually used in a way very much like the ordinary way we would use it, since Hume is there talking about the fictions of poets in particular.)
Since therefore the memory, is known, neither by the order of its complex ideas, nor the nature of its simple ones; it follows, that the difference betwixt it and the imagination lies in its superior force and vivacity. A man may indulge his fancy in feigning any past scene of adventures; nor would there be any possibility of distinguishing this from a remembrance of a like kind, were not the ideas of the imagination fainter and more obscure.
It is evident, that whatever is present to the memory, striking upon the mind with a vivacity, which resembles an immediate impression, must become of considerable moment in all the operations of the mind, and must easily distinguish itself above the mere fictions of the imagination. Of these impressions or ideas of the memory we form a kind of system, comprehending whatever we remember to have been present, either to our internal perception or senses; and every particular of that system, joined to the present impressions, we are pleased to call a reality.[...]
Mean while I shall carry my observation a step farther, and assert, that even where the related object is but feigned, the relation will serve to enliven the idea, and encrease its influence. A poet, no doubt, will be the better able to form a strong description of the Elysian fields, that he prompts his imagination by the view of a beautiful meadow or garden; as at another time he may by his fancy place himself in the midst of these fabulous regions, that by the feigned contiguity he may enliven his imagination.
But though I cannot altogether exclude the relations of resemblance and contiguity from operating on the fancy in this manner, it is observable that, when single, their influence is very feeble and uncertain. As the relation of cause and effect is requisite to persuade us of any real existence, so is this persuasion requisite to give force to these other relations. For where upon the appearance of an impression we not only feign another object, but likewise arbitrarily, and of our mere good-will and pleasure give it a particular relation to the impression, this can have but a small effect upon the mind; nor is there any reason, why, upon the return of the same impression, we should be determined to place the same object in the same relation to it. There is no manner of necessity for the mind to feign any resembling and contiguous objects; and if it feigns such, there is as little necessity for it always to confine itself to the same, without any difference or variation. And indeed such a fiction is founded on so little reason, that nothing but pure caprice can determine the mind to form it; and that principle being fluctuating and uncertain, it is impossible it can ever operate with any considerable degree of force and constancy. The mind forsees and anticipates the change; and even from the very first instant feels the looseness of its actions, and the weak hold it has of its objects. And as this imperfection is very sensible in every single instance, it still encreases by experience and observation, when we compare the several instances we may remember, and form a general rule against the reposing any assurance in those momentary glimpses of light, which arise in the imagination from a feigned resemblance and contiguity.
I have already observd, that time, in a strict sense, implies succession, and that when we apply its idea to any unchangeable object, it is only by a fiction of the imagination, by which the unchangeable object is supposd to participate of the changes of the co-existent objects, and in particular of that of our perceptions. This fiction of the imagination almost universally takes place; and it is by means of it, that a single object, placd before us, and surveyd for any time without our discovering in it any interruption or variation, is able to give us a notion of identity.[...]
The smooth passage of the imagination along the ideas of the resembling perceptions makes us ascribe to them a perfect identity. The interrupted manner of their appearance makes us consider them as so many resembling, but still distinct beings, which appear after certain intervals. The perplexity arising from this contradiction produces a propension to unite these broken appearances by the fiction of a continued existence[...]
The supposition of the continued existence of sensible objects or perceptions involves no contradiction. We may easily indulge our inclination to that supposition. When the exact resemblance of our perceptions makes us ascribe to them an identity, we may remove the seeming interruption by feigning a continued being, which may fill those intervals, and preserve a perfect and entire identity to our perceptions.[...]
But as we here not only feign but believe this continued existence, the question is, from whence arises such a belief...
Our memory presents us with a vast number of instances of perceptions perfectly resembling each other, that return at different distances of time, and after considerable interruptions. This resemblance gives us a propension to consider these interrupted perceptions as the same; and also a propension to connect them by a continued existence, in order to justify this identity, and avoid the contradiction, in which the interrupted appearance of these perceptions seems necessarily to involve us. Here then we have a propensity to feign the continued existence of all sensible objects; and as this propensity arises from some lively impressions of the memory, it bestows a vivacity on that fiction: or in other words, makes us believe the continued existence of body.[...]
It is indeed evident, that as the vulgar suppose their perceptions to be their only objects, and at the same time believe the continued existence of matter, we must account for the origin of the belief upon that supposition. Now upon that supposition, it is a false opinion that any of our objects, or perceptions, are identically the same after an interruption; and consequently the opinion of their identity can never arise from reason, but must arise from the imagination. The imagination is seduced into such an opinion only by means of the resemblance of certain perceptions; since we find they are only our resembling perceptions, which we have a propension to suppose the same. This propension to bestow an identity on our resembling perceptions, produces the fiction of a continued existence; since that fiction, as well as the identity, is really false, as is acknowledged by all philosophers, and has no other effect than to remedy the interruption of our perceptions, which is the only circumstance that is contrary to their identity.[...]
The imagination tells us, that our resembling perceptions have a continued and uninterrupted existence, and are not annihilated by their absence. Reflection tells us, that even our resembling perceptions are interrupted in their existence, and different from each other. The contradiction betwixt these opinions we elude by a new fiction, which is conformable to the hypotheses both of reflection and fancy, by ascribing these contrary qualities to different existences; the interruption to perceptions, and the continuance to objects. Nature is obstinate, and will not quit the field, however strongly attacked by reason; and at the same time reason is so clear in the point, that there is no possibility of disguising her. Not being able to reconcile these two enemies, we endeavour to set ourselves at ease as much as possible, by successively granting to each whatever it demands, and by feigning a double existence, where each may find something, that has all the conditions it desires.
In order to reconcile which contradictions the imagination is apt to feign something unknown and invisible, which it supposes to continue the same under all these variations; and this unintelligible something it calls a substance, or original and first matter.[...]
But the mind rests not here. Whenever it views the object in another light, it finds that all these qualities are different, and distinguishable, and separable from each other; which view of things being destructive of its primary and more natural notions, obliges the imagination to feign an unknown something, or original substance and matter, as a principle of union or cohesion among these qualities, and as what may give the compound object a title to be called one thing, notwithstanding its diversity and composition.
In our arrangement of bodies we never fail to place such as are resembling, in contiguity to each other, or at least in correspondent points of view: Why? but because we feel a satisfaction in joining the relation of contiguity to that of resemblance, or the resemblance of situation to that of qualities. The effects this propensity have been already observed in that resemblance, which we so readily suppose betwixt particular impressions and their external causes. But we shall not find a more evident effect of it, than in the present instance, where from the relations of causation and contiguity in time betwixt two objects, we feign likewise that of a conjunction in place, in order to strengthen the connexion.
Our propensity to this mistake is so great from the resemblance above-mentioned, that we fall into it before we are aware; and though we incessantly correct ourselves by reflection, and return to a more accurate method of thinking, yet we cannot long sustain our philosophy, or take off this biass from the imagination. Our last resource is to yield to it, and boldly assert that these different related objects are in effect the same, however interrupted and variable. In order to justify to ourselves this absurdity, we often feign some new and unintelligible principle, that connects the objects together, and prevents their interruption or variation. Thus we feign the continued existence of the perceptions of our senses, to remove the interruption: and run into the notion of a soul, and self, and substance, to disguise the variation. But we may farther observe, that where we do not give rise to such a fiction, our propension to confound identity with relation is so great, that we are apt to imagine something unknown and mysterious, connecting the parts, beside their relation; and this I take to be the case with regard to the identity we ascribe to plants and vegetables.[...]
What I have said concerning the first origin and uncertainty of our notion of identity, as applied to the human mind, may be extended with little or no variation to that of simplicity. An object, whose different co-existent parts are bound together by a close relation, operates upon the imagination after much the same manner as one perfectly simple and indivisible and requires not a much greater stretch of thought in order to its conception. From this similarity of operation we attribute a simplicity to it, and feign a principle of union as the support of this simplicity, and the center of all the different parts and qualities of the object.
Treatise 2.2.3 (footnote):
It is a quality, which I have already observed in human nature, that when two objects appear in a close relation to each other, the mind is apt to ascribe to them any additional relation, in order to compleat the union; and this inclination is so strong, as often to make us run into errors (such as that of the conjunction of thought and matter) if we find that they can serve to that purpose. Many of our impressions are incapable of place or local position; and yet those very impressions we suppose to have a local conjunction with the impressions of sight and touch, merely because they are conjoined by causation, and are already united in the imagination. Since, therefore, we can feign a new relation, and even an absurd one, in order to compleat any union, it will easily be imagined, that if there be any relations, which depend on the mind, it will readily conjoin them to any preceding relation, and unite, by a new bond, such objects as have already an union in the fancy. Thus for instance, we never fail, in our arrangement of bodies, to place those which are resembling in contiguity to each other, or at least in correspondent points of view; because we feel a satisfaction in joining the relation of contiguity to that of resemblance, or the resemblance of situation to that of qualities. And this is easily accounted for from the known propertes of human nature.
The difficulties, that occur to us, in supposing a moral obligation to attend promises, we either surmount or elude. For instance; the expression of a resolution is not commonly supposed to be obligatory; and we cannot readily conceive how the making use of a certain form of words should be able to cause any material difference. Here, therefore, we feign a new act of the mind, which we call the willing an obligation; and on this we suppose the morality to depend. But we have proved already, that there is no such act of the mind, and consequently that promises impose no natural obligation.