1. Epicurus
1.1. Principal Doctrines
1.1.1. PD22. We must consider both the real purpose, and all the evidence of direct perception, to which we always refer the conclusions of opinion; otherwise, all will be full of doubt and confusion.
1.1.2. PD23. If you fight against all sensations, you will have no standard by which to judge even those of them which you say are false.
1.1.3. PD24. If you reject any single sensation, and fail to distinguish between the conclusion of opinion, as to the appearance awaiting confirmation, and that which is actually given by the sensation or feeling, or each intuitive apprehension of the mind, you will confound all other sensations, as well, with the same groundless opinion, so that you will reject every standard of judgment. And if among the mental images created by your opinion you affirm both that which awaits confirmation, and that which does not, you will not escape error, since you will have preserved the whole cause of doubt in every judgment between what is right and what is wrong.
1.1.4. PD25. If on each occasion, instead of referring your actions to the end of nature, you turn to some other, nearer, standard, when you are making a choice or an avoidance, your actions will not be consistent with your principles.
1.2. Epicurus, Letter to Pythocles
[Line 94, translation by ____] The wanings of the moon and its subsequent waxings ..... may be explained in all the ways in which phenomena on earth invite us to such explanations of these phases, if only one does not fall in love with the method of a single explanation (μοναχῇ τρόπος ) and groundlessly disapproves of others, without having considered what it is possible for a human being to observe and what it is not and, for this reason, desirous of observing things that cannot be observed’.
1.3. Epicurus, On Nature XI
[Line Ia11-19; Sedley] The sun, if we walk towards the place from which it appeared to us] to rise, directing ourselves up into the mainland zone, appears to us to set where we previously passed by, sometimes even when we have moved in all only a short distance. And this time we cannot blame it on the latitudinal movements. Why after all should you declare the measurement from here, or the one from here, or the one from here, or this one a more reliable guide of the risings and settings (of the sun)?
1.4. Epicurus, On Nature XI
[Line IIa1 - 21; Sedley] They cannot hope] to form a [mental] model ([ὁ]μοίωμα) and to reason out (συλλογίζεσθαι) anything about these matters. For it seems to me that when they spend their time contriving some of them (I means their [ὄρ]γανα, instruments) and fooling around with others, it is no wonder, in view not only of the enslavements brought upon them by their doctrines but also (as far as concerns the appearances of the sun) of the indeterminacies (ἀοριστείας) of risings and settings, that they cannot form an adequate mental model by means of their instruments which produce no regularity. But their instruments are ...
2. Lucretius
2.1. Lucretius Book IV
[Line no. - Cyril Bailey] Again, if any one thinks that nothing is known, he knows not whether that can be known either, since he admits that he knows nothing. Against him then I will refrain from joining issue, who plants himself with his head in the place of his feet. And yet were I to grant that he knows this too, yet I would ask this one question; since he has never before seen any truth in things, whence does he know what is knowing, and not knowing each in turn, what thing has begotten the concept of the true and the false, what thing has proved that the doubtful differs from the certain? You will find that the concept of the true is begotten first from the senses, and that the senses cannot be gainsaid. For something must be found with a greater surety, which can of its own authority refute the false by the true. Next then, what must be held to be of greater surety than sense? Will reason, sprung from false sensation, avail to speak against the senses, when it is wholly sprung from the senses? For unless they are true, all reason too becomes false. Or will the ears be able to pass judgement on the eyes, or touch on the ears? or again will the taste in the mouth refute this touch; will the nostrils disprove it, or the eyes show it false? It is not so, I trow. For each sense has its faculty set apart, each its own power, and so it must needs be that we perceive in one way what is soft or cold or hot, and in another the diverse colours of things, and see all that goes along with colour. Likewise, the taste of the mouth has its power apart; in one way smells arise, in another sounds. And so it must needs be that one sense cannot prove another false. Nor again will they be able to pass judgement on themselves, since equal trust must at all times be placed in them.
Therefore, whatever they have perceived on each occasion, is true. And if reason is unable to unravel the cause, why those things which close at hand were square, are seen round from a distance, still it is better through lack of reasoning to be at fault in accounting for the causes of either shape, rather than to let things clear seen slip abroad from your grasp, and to assail the grounds of belief, and to pluck up the whole foundations on which life and existence rest. For not only would all reasoning fall away; life itself too would collapse straightway, unless you chose to trust the senses, and avoid headlong spots and all other things of this kind which must be shunned, and to make for what is opposite to these. Know, then, that all this is but an empty store of words, which has been drawn up and arrayed against the senses. Again, just as in a building, if the first ruler is awry, and if the square is wrong and out of the straight lines, if the level sags a whit in any place, it must needs be that the whole structure will be made faulty and crooked, all awry, bulging, leaning forwards or backwards, and out of harmony, so that some parts seem already to long to fall, or do fall, all betrayed by the first wrong measurements; even so then your reasoning of things must be awry and false, which all springs from false senses.
3. Diogenes of Oinoanda
[Line No. __ Martin Ferguson Smith] [Others do not] explicitly [stigmatise] natural science as unnecessary, being ashamed to acknowledge [this], but use another means of discarding it. For, when they assert that things are inapprehensible, what else are they saying than that there is no need for us to pursue natural science? After all, who will choose to seek what he can never find?
Now Aristotle and those who hold the same Peripatetic views as Aristotle say that nothing is scientifically knowable, because things are continually in flux and, on account of the rapidity of the flux, evade our apprehension. We on the other hand acknowledge their flux, but not its being so rapid that the nature of each thing [is] at no time apprehensible by sense-perception. And indeed [in no way would the upholders of] the view under discussion have been able to say (and this is just what they do [maintain] that [at one time] this is [white] and this black, while [at another time] neither this is [white nor] that black, [if] they had not had [previous] knowledge of the nature of both white and black.