It seems like a paradox, but only from the Western philosophical idea of 'free will.' Classical Theravada, on the other hand, views free will more or less as an illusion, and instead takes a more casually determined view of volition.
Conventionally speaking, we appear to have functional choice via intention operating within a broader framework of causality that conditions the choices available to us at any given time. However, on a deeper level, intention itself is a product of the aggregate of mental formations (sankharakhandha). Therefore, being a product or process within one of the aggregates, which themselves are types of processes and not-self (anatta), this type of internal decision maker or will-to-do, if you will, has its own requisite conditions and is also not-self, since whatever is conditioned and subject to change can't be said to have an unchanging essence or being.
In essence, volition itself isn't an illusion, it's simply not the result of an independent agent or self; and it, like everything else in the world, is ultimately the result causally determined processes. True free will requires an independent agent, and Buddhism effectively rejects such an agency. And while I tend to take a more moderate position myself, Buddhism is entirely compatible with causal determinism.
For example, Dhammanando Bhikkhu once gave me the example of a mosquito biting you on the nose: first you feel annoyed and want to squash it, but then you recall that you're a precept-observing Buddhist and so restrain yourself.
He explained that when this event is described in conventional terms, or according to the Sutta method, it might be said that you had a choice to kill the mosquito or to refrain, and that you chose the latter. But when it's described according to the Abhidhamma method, your abstention from killing wasn't due to choice but to the arising of kusala cetasikas (wholesome mental factors) such as moral shame and fear of wrong-doing (hiri & ottappa), and abstinence (virati), i.e., it was causally determined.
And then there are passages like this from the Dhammasangani (pp. 7-8):
What on that occasion is volition (cetana)? The volition, purpose, purposefulness, which is born of contact with the appropriate element of representative intellection - that is the volition that there then is.
And the Atthasalini, pp.147-148:
Volition is that which co-ordinates, that is, it binds closely (abhisandahati) to itself associated states as objects. This is its characteristic; its function is conation. There is no such thing as volition in the four planes of existence without the characteristic of co-ordinating; all volition has it. But the function of conation is only in moral and immoral states; as regards activity in moral and immoral acts, the remaining associated states play only a restricted part. But volition is exceedingly energetic. It makes double effort, double exertion. Hence the Ancients said: 'Volition is like the nature of a landowner, a cultivator who, taking fifty-five strong men, went down to the fields to reap. He was exceedingly energetic and exceedingly strenuous; he doubled his strength, he doubled his effort, and said, "Take your sickles," and so forth, pointed out the portion to be reaped, offered them drink, food, scent, flowers, etc., and took an equal share of the work.' Volition is like the cultivator; the fifty-five moral states which arise as factors of consciousness are like the fifty-five strong men; like the time of doubling strength, doubling effort by the cultivator is the doubled strength, double effort of volition as regards activity in moral and immoral acts. Thus should conation as its function be understood.
It has directing as manifestation. It arises directing associated states, like the chief disciple, the chief carpenter, etc., who fulfil their own and others' duties... even so, when volition starts work on its object, it sets associated states to do each its own work. For when it puts forth energy, they also put forth energy... It is also evident that it arises by causing associated states to be energetic in such things as recollecting an urgent work and so forth.
Incidentally, I think the Abhidhammic position accords well with what neuroscientist Sam Harris writes about the illusion of free will here, here and here.
