Epicurean Free Will
The texts are clear that Epicurus held that it was important to affirm that within natural limits we have the power to make choices that affect our futures. (See references below.) But many people ask “how do I know this to be true?” (“if I am a scientist wanting to validate or invalidate a hypothesis…)
There are tremendous numbers of arguments that can be employed on both sides, but here is the way I look at it – I first go back to the issue of knowledge, and to the Epicurean framework for determining truth based on the senses, anticipations, and pain and pleasure. Using those faculties, we observe that it appears to us that we (and other higher animals) can choose all sorts of things. This ability appears validated by the senses (and by the other legs too, I think) and so we have a starting point that we are observing something that is firmly established by all the sensory evidence available to us.
To me, the issue then goes back to one of the core issues that separates the Epicurean canon and the Platonic/Aristotelian worship of reason as above the senses. Sure it is possible to assert premises and theories that everything is just an illusion and we are just robots with all our thoughts predetermined. We can assert that with our “logic” but do we have any proof of it through the senses and through observation?
Whenever clear and firm evidence of the senses establishes something (and all human experience establishes that we have at least limited free will) then if we are devoted to “true reason” (reason which is based on evidence) it is a fallacy to seriously entertain a logical theory which is not only based on no evidence, but contradicts all the firm evidence of our experience. To do so would be to “reject any single sensation” (actually it would be to reject multitudes of sensations) which is pointed out as fallacious in PD24.
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.”
Once we throw out the observations/sensations that we have at least limited free will, then we have committed the error of PD24 because we are elevating abstract logic over the observable facts of reality. Logic without facts to support it is fantasy, but some people still insist on dignifying that sort of fantasy (elaborate constructs which are self-consistent but not verified by reality) with the name “logic.”
This is a very complicated subject, and much more needs to be said on it, but in closing this post I would cite the following section from Lucretius Book 2, where you see Lucretius responding to the deterministic argument by deducing free will from his observation of the horse. The point is that he is meeting the argument by OBSERVATION, not by accepting the unverifiable premises of the deterministic argument:
“Once again, if every motion is always linked on, and the new always arises from the old in order determined, nor by swerving do the first-beginnings make a certain start of movement to break through the decrees of fate, so that cause may not follow cause from infinite time; whence comes this free will for living things all over the earth, whence, I ask, is it wrested from fate, this will whereby we move forward, where pleasure leads each one of us, and swerve likewise in our motions neither at determined times nor in a determined direction of place, but just where our mind has carried us? For without doubt it is his own will which gives to each one a start for this movement, and from the will the motions pass flooding through the limbs. Do you not see too how, when the barriers are flung open, yet for an instant of time the eager might of the horses cannot burst out so suddenly as their mind itself desires?”
Also, as a final point, check this reference from Lucretius Book 1. The point here is that unless we have confidence in the clear evidence of our senses, it is useless to attempt to “reason”:
“But now, to weave again at the web, which is the task of my discourse, all nature then, as it is of itself, is built of these two things: for there are bodies and the void, in which they are placed and where they move hither and thither. For that body exists is declared by the feeling which all share alike; and unless faith in this feeling be firmly grounded at once and prevail, there will be naught to which we can make appeal about things hidden, so as to prove aught by the reasoning of the mind. “*
Here are a number of key references in the Epicurean texts to “free will”:
Letter to Menoeceus:
Fate, which some introduce as sovereign over all things, he scorns, affirming rather that some things happen of necessity, others by chance, others through our own agency. For he sees that necessity destroys responsibility and that chance is inconstant; whereas our own actions are autonomous, and it is to them that praise and blame naturally attach. It were better, indeed, to accept the legends of the gods than to bow beneath that yoke of destiny which the natural philosophers have imposed. The one holds out some faint hope that we may escape if we honor the gods, while the necessity of the naturalists is deaf to all entreaties. Nor does he hold chance to be a god, as the world in general does, for in the acts of a god there is no disorder; nor to be a cause, though an uncertain one, for he believes that no good or evil is dispensed by chance to men so as to make life blessed, though it supplies the starting-point of great good and great evil. He believes that the misfortune of the wise is better than the prosperity of the fool. It is better, in short, that what is well judged in action should not owe its successful issue to the aid of chance.
Vatican Sayings: Necessity is an evil; but there is no necessity for continuing to live with necessity.  (PD 29) Of our desires some are natural and necessary, others are natural but not necessary; and others are neither natural nor necessary, but are due to groundless opinion.  We must not force Nature but persuade her. We shall persuade her if we satisfy the necessary desires and also those bodily desires that do not harm us while sternly rejecting those that are harmful.  He who asserts that everything happens by necessity can hardly find fault with one who denies that everything happens by necessity; by his own theory this very argument is voiced by necessity.  We must free ourselves from the prison of public education and politics.  Freedom is the greatest fruit of self-sufficiency.
Many many others have the inference, like VS 58 and 77, that we have the free will to take action, otherwise they are meaningless.
The entire system of ethics described in Diogenes Laertius presupposes free will, stated here as the ability to choose and avoid:
“But as to the conduct of life, what we ought to avoid and what to choose, he writes as follows. Before quoting his words, however, let me go into the views of Epicurus himself and his school concerning the wise man.
Diogenes Laertius records the Epicurus rejected ANY attempt to predict the future:
Elsewhere he rejects the whole of divination, as in the short epitome, and says, “No means of predicting the future really exists, and if it did, we must regard what happens according to it as nothing to us.”
Lucretius Book 2:
Once again, if every motion is always linked on, and the new always arises from the old in order determined, nor by swerving do the first-beginnings make a certain start of movement to break through the decrees of fate, so that cause may not follow cause from infinite time; whence comes this free will for living things all over the earth, whence, I ask, is it wrested from fate, this will whereby we move forward, where pleasure leads each one of us, and swerve likewise in our motions neither at determined times nor in a determined direction of place, but just where our mind has carried us? For without doubt it is his own will which gives to each one a start for this movement, and from the will the motions pass flooding through the limbs. Do you not see too how, when the barriers are flung open, yet for an instant of time the eager might of the horses cannot burst out so suddenly as their mind itself desires?