Quotations about   agency

Therefore virtue also depends on ourselves. And so also does vice. For where we are free to act we are also free to refrain from acting, and where we are able to say No we are also able to say Yes; if therefore we are responsible for doing a thing when to do it is right, we are also responsible for not doing it when not to do it is wrong, and if we are responsible for rightly not doing a thing, we are also responsible for wrongly doing it. But if it is in our power to do and to refrain from doing right and wrong, and if, as we saw, being good or bad is doing right or wrong, it consequently depends on us whether we are virtuous or vicious.

Aristotle (384-322 BC) Greek philosopher
Nicomachean Ethics [Ἠθικὰ Νικομάχεια], Book 3, ch. 5 (3.5) / 1113b (c. 325 BC) [tr. Rackham (1934)]
    (Source)

Source of the common summary, "What lies in our power to do, it lies in our power not to do." Alternate translations:

Virtue is in our power. And so too is Vice: because wherever it is in our power to do it is also in our power to forbear doing, and vice versâ: therefore if the doing (being in a given case creditable) is in our power, so too is the forbearing (which is in the same case discreditable), and vice versâ. But if it is in our power to do and to forbear doing what is creditable or the contrary, and these respectively constitute the being good or bad, then the being good or vicious characters is in our power.
[tr. Chase (1847), ch. 7]

Virtue is in our own power, and, by parity of reasoning, so is vice. For where it is in our power to do a thing, it is equally in our power to abstain from doing it; where refusal is in our power, assent is equally so. So that, if to do such or such a thing, which is noble, be in our power, to abstain from it, which is disgraceful, will be equally in our power; and ift0o abstain from doing such or such a thing, which is noble, be in our power, then to do it, which is disgraceful, will be equally in our power. And if, in a word, it be in our power to do what is noble and what is disgraceful, it is equally in our power not to do it. Or in other words, it is in our power to be good men or bad. It rests, then, with ourselves whether we are to be virtuous or vicious.
[tr. Williams (1869)]

Virtue and vice are both alike in our own power; for where it is in our power to act, it is also in our power to refrain from acting, and where it is our power to refrain from acting, it is also in our power to act. Hence if it is in our power to act when action is noble, it will also be in our power to refrain from acting when inaction is shameful, and if it is our power to refrain from acting when inaction is noble, it will also be in our power to act when action is shameful. But if it is in our power to do, and likewise not to do, what is noble and shameful, and if so to act or not to act is as we have seen to be good or bad, it follows that it is in our power to be virtuous or vicious.
[tr. Welldon (1892), ch. 7]

Therefore virtue depends upon ourselves: and vice likewise. For where it lies with us to do, it lies with us not to do. Where we can say no, we can say yes. If then the doing a deed, which is noble, lies with us, the not doing it, which is disgraceful, lies with us; and if the not doing, which is noble, lies with us, the doing, which is disgraceful, also lies with us. But if the doing and likewise the not doing of noble or base deeds lies with us, and if this is, as we found, identical with being good or bad, then it follows that it lies with us to be worthy or worthless men.
[tr. Peters (1893)]

Therefore virtue also is in our own power, and so too vice. For where it is in our power to act it is also in our power not to act, and vice versa; so that, if to act, where this is noble, is in our power, not to act, which will be base, will also be in our power, and if not to act, where this is noble, is in our power, to act, which will be base, will also be in our power. Now if it is in our power to do noble or base acts, and likewise in our power not to do them, and this was what being good or bad meant, then it is in our power to be virtuous or vicious.
[tr. Ross (1908)]

Virtue too is up to us, then, and similarly, vice. For where acting is up to us, so is not acting, and where saying "No" is up to us, so is saying "Yes." Hence if acting, when it is noble, is up to us, not acting, when it is shameful, will also be up to us. And if not acting, when it is noble, is up to us, acting, when it is shameful, will also be up to us. But if doing noble actions or doing shameful ones is up to us, and similarly, also not doing them (which is what being good people and being bad people consisted in), then being decent or base will be up to us.
[tr. Reeve (1948)]

So virtue, too, is in our power, and also vice for a similar reason. For where it is in our power to act, it is also in our power not to act, and where it is in our power not to act, it is also in our power to act; so if to act, when it is noble, is in our power, then also not to act, which would then be disgraceful, would be in our power, and if not to act, when it is noble, is in our power, then also to act, which would then be disgraceful, would be in our power. If it is in our power, then, to do what is noble or disgraceful, and likewise not to do what is noble or disgraceful, and to act or not to act nobly or disgracefully, as stated earlier, is to be good or bad, then it is our power to be good or bad men.
[tr. Apostle (1975)]

Therefore virtue lies in our power, and similarly so does vice; because where it is in our power to act, it is also in our power not to act, and where we can refuse we can also also comply. So if is in our power to do a thing when it is right, it will also be in our power not to do it when it is wrong; and if it is in our power not to do it when it is right, it will also be in our power to do it when it is wrong. And if it is in our power to do right and wrong, and similarly not to do them; and if, as we saw, doing right or wrong is the essence of being good or bad, it follows that it is in our power to be decent or worthless.
[tr. Thomson/Tredennick (1976)]

Virtue, then, is in our power, and so is vice. Where it is in our power to act, it is also in our power not to act, and where saying "No" is in our power, so is saying "Yes" so that if it is in our power to act when it would be noble, it will also be in our power not to act when it would be shameful, and if it is in our power not to act when it would be noble, it will also be in our power to act when it would be shameful. Now if it is in our power to do noble and shameful actions, and the same goes for not doing them, and if, as we saw, being good and bad consists in this, then it is in our power to be good or bad.
[tr. Crisp (2000)]