In case it may be helpful here are other references in Lucretius which Bailey has translated the phrase "clear to see":
 And yet we must not think that all particles can be linked together in all ways, for you would see monsters created everywhere, forms coming to being half man, half beast, and sometimes tall branches growing out from a living body, and many limbs of land-beasts linked with beasts of the sea, and nature too throughout the lands, that are the parents of all things, feeding Chimaeras breathing flame from their noisome mouths. But it is clear to see that none of these things comes to be, since we see that all things are born of fixed seeds and a fixed parent, and can, as they grow, preserve their kind. You may be sure that that must needs come to pass by a fixed law. For its own proper particles separate from every kind of food and pass within into the limbs of everything, and are there linked on and bring about the suitable movements. But, on the other hand, we see nature cast out alien matter on to the ground, and many things with bodies unseen flee from the body, driven by blows, which could not be linked to any part nor within feel the lively motions in harmony with the body and imitate them.
 \[Yet many wise men have thought\] that the sensation of the mind is not placed in any part determined, but is a certain vital habit of the body, which the Greeks call a harmony, in that it makes us live with sensation, although in no part does an understanding exist; as when often good health is said to belong to the body, and yet it is not itself any part of a healthy man. In this wise they do not set the sensation of the mind in any part determined; and in this they seem to me to wander very far astray. Thus often the body, which is clear to see, is sick, when, all the same we feel pleasure in some other hidden part; and contrariwise it happens that the reverse often comes to be in turn, when one wretched in mind feels pleasure in all his body; in no other wise than if, when a sick man’s foot is painful, all the while, may be, his head is in no pain. Moreover, when the limbs are given up to soft sleep, and the heavy body lies slack and senseless, yet there is something else in us, which at that very time is stirred in many ways, and admits within itself all the motions of joy and baseless cares of heart.
 This nature then of the soul is protected by the whole body, and is itself the guardian of the body, and the cause of its life; for the two cling together by common roots, and it is seen that they cannot be torn asunder without destruction. Even as it is not easy to tear out the scent from lumps of frankincense, but that its nature too passes away. So it is not easy to draw out the nature of mind and soul from the whole body, but that all alike is dissolved. With first-beginnings so closely interlaced from their very birth are they begotten, endowed with a life shared in common, nor, as is clear to see, can the power of body or mind feel apart, either for itself without the force of the other, but by the common motions of the two on this side and on that is sensation kindled and fanned throughout our flesh.
 The Sisyphus in our life too is clear to see, he who open-mouthed seeks from the people the rods and cruel axes, and evermore comes back conquered and dispirited. For to seek for a power, which is but in name, and is never truly given, and for that to endure for ever grinding toil, this is to thrust uphill with great effort a stone, which after all rolls back from the topmost peak, and headlong makes for the levels of the plain beneath.
 First of all, since among things clear to see many things give off bodies, in part scattered loosely abroad, even as wood gives off smoke and fires heat, and in part more closely knit and packed together, as when now and then the grasshoppers lay aside their smooth coats in summer, and when calves at their birth give off a caul from their outermost body, and likewise when the slippery serpent rubs off its vesture on the thorns; for often we see the brambles laden with these wind-blown spoils from snakes. And since these things come to pass, a thin image from things too must needs be given off from the outermost body of things. For why these films should fall and part from things any more than films that are thin, none can breathe a word to prove; above all, since on the surface of things there are many tiny bodies, which could be cast off in the same order wherein they stood, and could preserve the outline of their shape, yea, and be cast the more quickly, inasmuch as they can be less entangled, in that they are few, and placed in the forefront. For verily we see many things cast off and give out bodies in abundance, not only from deep beneath, as we said before, but often too from the surface, such as their own colour. And commonly is this done by awnings, yellow and red and steely-blue, when stretched over great theatres they flap and flutter, spread everywhere on masts and beams. For there they tinge the assembly in the tiers beneath, and all the bravery of the stage and the gay-clad company of the elders, and constrain them to flutter in their colours. And the more closely are the hoardings of the theatre shut in all around, the more does all the scene within laugh, bathed in brightness, as the light of day is straitened. Since then the canvas gives out this hue from its outermost body, each several thing also must needs give out thin likenesses, since in either case they are throwing off from the surface. There are then sure traces of forms, which fly about everywhere, endowed with slender bulk, nor can they be seen apart one by one.
 Lastly, what is there so marvellous in this, if the human race, with strong voice and tongue, should mark off things with diverse sounds for diverse feelings? When the dumb cattle, yea and the races of wild beasts are wont to give forth diverse unlike sounds, when they are in fear or pain, or again when their joys grow strong. Yea verily, this we may learn from things clear to see.
 It comes to pass, too, sometimes, that the force of a mighty wind rushing on tears through the cloud and breaks it asunder with a front attack. For what the blast can do there is shown by things clear to see here on earth, where the wind is gentler and yet it tears out and sucks up tall trees from their lowest roots.
 Nor is it the same as when we move forward impelled by a blow from the strong might and strong constraint of another. For then it is clear to see that all the matter of the body moves and is hurried on against our will, until the will has reined it back throughout the limbs. Do you not then now see that, albeit a force outside pushes many men and constrains them often to go forward against their will and to be hurried away headlong, yet there is something in our breast, which can fight against it and withstand it? And at its bidding too the store of matter is constrained now and then to turn throughout the limbs and members, and, when pushed forward, is reined back and comes to rest again.