23. If we resist the senses, we have nothing left to which we can refer, or by which we may judge, the falsehood of the senses which we condemn.

Alternate Translations: Bailey: If you fight against all sensations you will have no standard by which to judge even those of them which you say are false. Strodach: If you reject all sensations, you will not have any point of reference by which to judge even the ones you claim are false.

Lucretius, De Rerum Natura Book IV: [I] f a man contends that nothing can be known, he knows not whether this contention itself can be known, since he admits that he knows nothing. I will therefore decline to argue the question against him who places his head where his feet should be. And yet granting that this man knows his contention to be true, I would still ask this question: Since he has never yet seen any truth in any thing, how does he know what “knowing” and “not knowing” are? What has produced his knowledge of the difference between the true and the false, and between the doubtful and the certain? You will find that all knowledge of the true comes from the senses, and that the senses cannot be refuted. For anything which on its own can distinguish that which is false from that which is true must by nature possess a higher certainty than the thing which it judges. Well, then, what can fairly be accounted of higher certainty than the senses? Shall reasoning alone be able to contradict the sensations? No, not when reasoning is itself wholly reliant on the senses for its accuracy. If the evidence of the senses is not true, then all reasoning based on that evidence is rendered false. Are the ears able to take the eyes to task, or the sense of touch take the ears to task? Shall the sense of taste or smell or vision call into question the sense of touch? No, for each sense has its own separate and distinct office and power. … It therefore follows that no sense can refute any other. Nor can any sense take itself to task, since equal credit must be assigned to the evidence it produces at all times. What has at any time appeared true to each sense must be taken as a true sensation.

At times you may experience sensations which your reason is unable to explain – for example, why a tower close at hand is seen to be square, but when seen at a distance appears round. In such cases it is better, if you are at a loss for a reason to explain this, to admit that you do not know the truth of the matter, rather than to accept an explanation that makes no sense. If you accept as true a possibility that contradicts your senses, you have set the stage to let slip from your grasp all those other things which you know to be manifestly true. In so doing you will ruin the groundwork of all your beliefs, and wrench up all the foundations on which life and existence rest. For not only would all reason give way, but life itself would fall to the ground, unless you pursue the truth and choose to trust the senses, shunning the steep cliffs of life that must be avoided. All that host of words drawn out in array against the senses is quite without meaning. If, in the construction of a building, the measuring stick first applied by the builder is crooked, or his square is untrue and swerves from its straight lines, or if there is the slightest hitch in any part of his level, all the construction will turn out to be faulty, crooked, sloping, leaning forward or backward, without symmetry, so that some parts seem ready to fall, and others do fall, all ruined by the first erroneous measurements. So too, all reasoning of things which is not founded on the senses will prove to be distorted and false.

Lucretius De Rerum Natura Book I: To say that all things are fire, and that nothing really exists except fire, as one philosopher does, is clearly sheer insanity. For this man takes his stand on the side of the senses at the same time that he fights against the senses. His argument challenges the authority of the senses, on which rests all our convictions, even his own belief about this fire (as he calls it) that is known only to himself. For what he is saying is that he believes that the senses can truly perceive fire, but he does not believe they can perceive all other things, which are not a bit less clear! Now this is clearly as false as it is foolish, for to what shall we appeal to resolve the question? What more certain test can we apply but that of the senses to judge truth and falsehood? Why should anyone choose to abolish all other things and choose to leave only fire? Why not abolish fire, and hold that nature is composed of all other things? It would be equal madness to affirm either one or the other position.

Letter to Herodotus: [As we move to the consideration of phenomena such as the nature of sight and images], we must consider that there may be [various] manners in which things of this kind are produced. But we must never accept anything in these various possibilities which at all contradicts the senses, and [in evaluating these things] we must consider in what way the senses are exercised and the relationship that is established between the external objects we observe and ourselves.