I see that I am just short of a year since the time this subject last came to mind. Before I remembered that this thread existed, this time I collected the references to smoothness from the Daniel Browne edition (below).
This time "smooth motion" comes to mind in the context of how Torquatus conveys that no proposition can be more true than that pleasure equals the absence of pain, bringing within the term "pleasure" both (1) the agreeable experiences that are the result of stimulation and (2) those which are not the result of stimulation but of the normal healthy function of the organism.
Might it not be possible or even likely that the common denominator between these two types of experiences would be the "smooth motion" of those two categories? When "stimulated," pleasurable / agreeable feelings are those stimulations that prompt the intensification ("condensation?") of the existing smooth motion of the living thing. When not "stimulated," the normal pleasurable / non-painful condition of life consists in the normal healthy smooth-motion functioning of that living being. Pain is the opposite - pain is rough motion - disruption in smooth motion at any speed.
Ralph Nader had a well known book "Unsafe At Any Speed." Altering that title a little, pleasure is smooth motion at any speed, and pain is rough motion at any speed. Total absence of speed is death.
There seem to be plenty of citations within Lucretius to indicate that Epicurus tied pleasure to smooth motion. These would be useful in supporting an argument that generally speaking pleasure is (or arises from our awareness of) smooth motion. From that perspective it is equally proper to consider the regular smooth motion of the "un-stimulated" person to be pleasure just like it is pleasure when "stimulation" produces irregular and intensified but still smooth motion.
Smoothness and Pleasure
Lucretius Book 2 - Daniel Browne
[398] Thus it is that honey and milk pass in the mouth with a pleasing sensation over the tongue; on the contrary, the bitter juice of wormwood and sharp Centaury torment the palate with a loathsome taste. From whence you collect easily that those things which agreeably affect the sense are composed of particles smooth and round; and such again that seem rough and bitter are bound together by parts more hooked, and closer twined; and therefore they tear the way to our senses, and wound the body as they enter through the skin.
[408] In short, such things as are agreeable to our senses, and those that are rough and unpleasant to the touch, are opposite, and formed of a figure very different from one another; lest you should think perhaps that the grating sound of the whetting of a saw was made of parts equally smooth, without the soft notes of a lute, which the musician forms upon the strings, awaked, as it were, by the gentle strokes of his fingers.
Nor are you to suppose that the seeds are of the same form which strike upon our nerves of smell, when a filthy carcass is burning, or when the stage is fresh sprinkled with Cilician saffron, or the altar sweetens the air with the odor of Arabian incense.
And so in colors you must not imagine such as are agreeable and delight our eyes are composed of the same fashioned seeds with those which prick our sense, and force us to weep, or seem dark or ugly, and shocking in appearance to us; for whatever pleases and delights our senses cannot be composed but of smooth particles; and, on the contrary, things that are hurtful and harsh cannot be formed without seeds that are filthy and disagreeable.
There are other seeds, likewise, which you cannot properly call smooth, nor are altogether hooked, with their points bent, but are rather shaped with small ankles, a little jutting out, and may be said rather to tickle than to hurt the senses; such as the acid taste of the sweet sauce made of the Lees of wine, or the sweet sauce made of the sweetish-bitter root of Elecampane. Lastly, that burning heat, or freezing cold, being formed of seeds of different figures, do affect the body with different sensation our touch is evidence sufficient to evince.
For Touch, the Touch (blessed be the Gods above!) is a Sense of the Body, either when something from without enters through the pores, or something from within hurts us, as it forces its way out, or pleases, as the effect of venery tickles as it passes through, or when the seeds, by striking against each other, raise a tumult in the body, and in that agitation confound the Sense; and this you may soon experience, if you strike yourself in any part with a blow of your hand. It is necessary, therefore, that the Principles of Things should consist of figures very different in themselves, since they affect the Senses in so different a manner.
[444] Further, those things which appear to us hard and thick, must necessarily be joined together by particles more hooked among themselves, and be held close by branched seeds. In the first rank of these, you are to place the rocks of Adamant, that defy the force of blows, and solid flints, and the strength of hard iron, and brazen hinges, that creak under the weight of their gates.
But Liquids that consist of fluid bodies, must be formed of seeds more smooth and round; for their globular particles are not entangled among themselves, and their flowing motion rolls on forward with the greater Ease.
But lastly, all such Things which you observe instantly to scatter, and fly away as smoke, clouds, and flame, if they do not consist altogether of particles that are smooth and round, yet neither are they formed of hooked Seeds, and therefore may pierce through bodies, and penetrate into stones; nor do their particles nevertheless stick mutually to one another, as we observe the particles of thorns do. From thence you may easily conclude that they are not composed of hooked or entangled, but of acute Principles.
But because you see the same things are bitter and fluid, as the Sea-water, are you to wonder in the least at this; For what is fluid is formed of Principles that are smooth and round, but with these smooth and round seeds are mixed others that are sharp, and give pain. Yet there is no necessity that these sharp seeds should be hooked and twined together; it is sufficient that they be globous as well as rough, that they may be qualified to flow along in their proper Course, as well as to hurt the sense.And that you may the sooner believe that these sharp seeds are mixed with those that are smooth, from whence the body of the sea becomes salt, the way is to separate them, and consider them distinct; for the Sea-water grows sweet by being often filtered through the Earth, and so fills the ditches, where it becomes soft; for it leaves behind the pungent seeds of the rough salt, which are more inclined to stick as they pass along, than those particles that are globular and smooth.
Book 3 - Smoothness of Mind
[177] I shall now go on to explain clearly of what sort of body this mind consists, and of what principles it is formed. And first I say that the mind is composed of very subtle and minute seeds; that it is so, attend closely, and you will find that nothing is accomplished with so much speed as what the mind attempts, and proposes to execute. The Mind therefore is swifter in its motion than anything in nature we can see or conceive. But that which is so exceedingly quick to move must consist of the roundest and most minute seeds, that may be set a-going by the lightest impulse. So water is moved and disposed to flow by ever so little force, because it is composed of small and slippery seeds; but the nature of Honey is more tenacious, its moisture is more unactive, and its motion slower; its principles stick closer among themselves; and for this reason, because it consists of seeds not so smooth, so subtle, and so round. And thus a large heap of poppy seeds is blown away by the gentlest breath of wind, and scattered abroad; but no blast can shake a heap of stones or darts. Therefore the smoother and smaller the principles of bodies are, the more easily they are disposed to motion, and the heavier and rougher the seeds are, the more fixed and stable they remain.
Since therefore the nature of the mind is so exceedingly apt to move, it must needs consist of small, smooth, and round seeds; and your knowing this, my sweet youth, will be found of great use, and very seasonable for your future inquiries. ...
[231] Yet we are not to suppose this nature of the mind to be simple and unmixed; for a thin breath mingled with a warm vapor, forsakes the bodies of dying men; and this vapor draws the air along with it, for there can be no heat without air intermixed, and heat being in its nature rare, must needs have some seeds of air united with it. We find then the mind consists of three principles: of vapor, air, and heat; yet all these are not sufficient to produce sense: For we cannot conceive that either of these, or all of them united, can be the cause of sensible motions that may produce reason and thought.
And therefore a fourth nature must needs be added to these (and this indeed has no name at all) but nothing can be more apt to move, nothing more subtle than this, nor consist more of small smooth seeds; and this is what first raises a sensible motion through the body: this, as it is formed of the minutest particles, is first put into motion, then the heat, and the unseen vapor receive a motion from it, and then we are and so all the limbs are set a-going; then is the blood agitated, and all the bowels become sensible, and last of all, pleasure or pain is communicated to the bones and marrow. But no pain or any violent evil can pierce so far without disordering and setting the whole into confusion, so that there is no more place for life, and the parts of the soul fly away through the pores of the body. But this motion often stops upon the surface of the body, and then the soul remains whole, and the life is preserved.
Book 4
Inspired, I wander over the Muses seats, of difficult access, and yet untrod; I love to approach the purest springs, and thence to draw large draughts; I love to crop fresh flowers and make a noble garland for my head from thence, where yet the Muses never bound another's temples with a crown like mine. And first I write of lofty things, and strive to free the mind from the severest bonds of what men call religion; then my verse I frame so clear, although my theme by dark; seasoning my lines with the poetic sweets of fancy, and reason justifies the method; for as physicians when they would prevail on children to take down a bitter draught of wormwood, first tinge the edges of the cup with sweet and yellow honey, that so the children's unsuspecting age, at least their lips, may be deceived, and take the bitter juice; thus harmlessly betrayed, but not abused, by tasting thus they rather have their health restored: So I, because this system seems severe and harsh to such who have not yet discerned its truth, and the common herd are utterly averse to this philosophy, I thought it fit to show these rigid principles in verse, smooth and alluring, and tinge them, as it were, with sweet poetic honey, thus to charm your mind with my soft numbers till you view the nature of all things clearly, and perceive the usefulness and order they display.
...
And now, in what manner each of the other senses distinguishes its proper object is a subject of no great difficulty to explain. And first, sound and all voices are heard when they enter the ears, and strike with their bodies upon the sense; for we must allow that sound and voice are bodies, because they have power to make impression upon the sense; for the voice often scrapes the jaws, and the noise makes the windpipe rough as it passes through. When the seeds of words begin to hurry in a crowd through the narrow nerves, and to rush abroad, those vessels being full, the throat is raked and made hoarse, and the voice wounds the passage through which it goes into the air. There is no question then but voice and words consist of corporeal principles, because they affect and hurt the sense. You are likewise to observe how much a continual speaking, from morning to night, takes off from the body; how much it wears away from the very nerves and strength of the speaker, especially if it be delivered in the highest stretch of the voice. Of necessity therefore voice must be a body, because the speaker loses many parts from himself. The roughness then of the voice depends upon the roughness of the seeds, as the smoothness is produced from smooth seeds; nor are the seeds from the same figure that strike the ears when the trumpet sounds with grave and murmuring blasts, as when the sackbut rings with its hoarse noise, or swans in the cold vales of Helicon sing out with mournful notes their sweet complaint.
....
Nor is the account of the tongue and palate, by which we taste, a subject of greater nicety or more difficult to explain. And first, we perceive a taste in the mouth when we squeeze the juice from our food by chewing, as if we were to press a sponge full of water in our hands to make it dry; then the juice we draw out is spread over the pores of the palate, and through the crooked passages of the spongy tongue. When the seeds of this flowing juice are smooth, they gently touch, and affect all the moist and sweating surface of the tongue with sweet delight; but the seeds, the more rough and sharp they are, the more they stimulate and tear the sense. And then the pleasure of taste we feel no further than the palate; when the food is driven down through the jaws and divided among the limbs, the pleasure is gone; nor is it of any concern with what meat our bodies are nourished, if you can but digest what you eat, and separate it among the members, and preserve the moist tenor of the stomach.
I shall now account why, as we find, different sorts of food are agreeable to different palates; or why, what is sour and bitter to some seems to others exceeding sweet. In these cases the variety and difference are so great that what is food to one will prove sharp poison to another; and it happens that a serpent touched with the spittle of a man expires and bites himself to death.
Besides, to us Hellebore is strong poison, but goats it fattens, and is nourishment to quails; and to understand by what means this comes to pass, you must recollect what we observed before, that seeds of different kinds are mingled in the composition of all bodies.
And then all animals supported by food, as they differ in outward shape, and after their several kinds have a different form of body and limbs, so they consist of seeds of different figures, and since their seeds differ, the pores and passages which (as we said) were in all the parts, and in the mouth and palate itself, must differ likewise; some must be less, some greater, some with three, some with four squares; many round, and some with many corners in various manners: For as the frame of the seeds and their motions require, the pores must differ in their figure. The difference of the pores depends upon the texture of the seeds, and therefore what is sweet to one is bitter to another: It is sweet because the smoothest seeds gently enter into the pores of the palate; but the same food is bitter to another because the sharp and hooked particles pierce the jaws and wound the sense.
Book Six
[80] That the rules therefore of right reason may keep these evils at the greatest distance from us, though I have offered many things upon this subject before, yet much still remains to be observed, which I shall adorn with the smoothest verse.
