They Are Self-Refuting
For this sort of account [Determinism] is self-refuting, and can never prove that everything is of the kind called 'necessitated'; but [the proponent of Determinism] debates this very question on the assumption that his opponent is himself responsible for talking nonsense. And even if he goes on to infinity saying that this action of his is in turn necessitated, always appealing to arguments, he is not reasoning it empirically so long as he goes on imputing to himself the responsibility for having reasoned correctly and to his opponent that for having reasoned incorrectly. But unless he were to stop attributing his action to himself and to pin it on necessity, he would not even be consistent. On the other hand, if [he uses] the word 'necessity' of that which we call our own agency, he is merely changing a name.... [Epicurus On Nature, D.S. translation]
Lastly, if anyone thinks that he knows nothing, he cannot be sure that he knows this, when he confesses that he knows nothing at all. I shall avoid disputing with such a trifler, who perverts all things, and like a tumbler with his head prone to the earth, can go no otherwise than backwards. And yet allow that he knows this, I would ask (since he had nothing before, to lead him into such a knowledge) from whence he had the notion what it was to know, or not to know; what was it that gave him an idea of Truth or Falsehood, and what taught him to distinguish between doubt and certainty? [Lucretius, On the Nature of Things, Book 4, Daniel Browne edition]
They Are Impossible to Live By In Practice
The first men to give a satisfactory account of causes ... turned a blind eye to themselves ... in order to hold necessity and accident responsible for everything. Indeed, the actual account promoting this view came to grief when it left [them] blind to the fact that in [their] actions [they] were clashing with [their] doctrine; and that if it were not that a certain blindness to the doctrine took hold of them while acting, they would be constantly perplexing themselves; and that wherever the doctrine prevailed, they would be falling into desperate calamities, while wherever it did not they would be filled with conflict because of the contradiction between their actions and their doctrine. [Epicurus On Nature, D.S. translation]
And though reason is not able to assign a cause why an object that is really four-square when near, should appear round when seen at a distance; yet, if we cannot explain this difficulty, it is better to give any solution, even a false one, than to deliver up all Certainty out of our power, to break in upon our first principle of belief, and tear up all foundations upon which our life and security depend. For not only all reason must be overthrown, but life itself must be immediately extinguished, unless you give credit to your senses. These direct you to fly from a precipice and other evils of this sort which are to be avoided, and to pursue what tends to your security. All therefore is nothing more than an empty parade of words that can be offered against the certainty of sense. [Lucretius, On the Nature of Things, Book 4, Daniel Browne edition]
