The Epicureans, especially, held that not all events could be fully determined by the course of previous events. In fact, the Epicureans believed that not only were there genuinely random events, but that if there such random events did not exist inside our minds or souls, we could never be free and responsible because we would be fated to do what we do.
This debate was revived in the Middle Ages and in early modern times in the theological form: how is freedom and responsibility possible if God plans everything down to the last detail? Then in more recent times it has reappeared in a nonreligious guise: how is freedom and moral responsibility possible if nature is a fully deterministic system (as many scientists insisted before the current popularity of quantum mechanics)?
I know it's a staple of Introduction to Philosophy courses, but after years of dealing with this problem on and off, I have come to the conclusion (not exactly by way of any knock-down arguments, if there are any) that this problem of freedom and determinism is a pseudo-problem.
First of all, determinism (the view that all events are caused in such a way that they could not have turned out otherwise) and indeterminism (the view that there is at least one exception that disproves determinism) are both metaphysical questions in the bad sense of the term. The debate is completely unresolvable empirically, like the question whether there is a God who transcends not only space and time but human experience as well. Immanuel Kant saw this very clearly in his Critique of Pure Reason.
Determinism is not provable because, although we can know better and better how particular phenomena are caused by antecedent events, we will probably never know them, or the natural laws that govern them, well enough to know that, given the precise condition of the universe at some earlier time t0, the phenomenon in question just had to occur at t1, with no chance at all of some alternative occurring in its stead.
Moreover, determinism is not provable because it would only take one truly random event to make determinism false, and short of being an all-knowing deity myself, there is no way I could know that such an event does not or did not exist.
Indeterminism is not provable because for any event that seems to be without a cause sufficient to produce it, it is possible that we lack sufficiently sophisticated measurement tools to detect its cause or a sufficiently advanced scientific theory to connect its cause to the event. Thus the appearance of indeterminism may be a reflection of our ignorance, not of the "fact" that there are random events.
But our experience of freedom and responsibility is not the experience of a random or an uncaused event. It has far more to do with knowledge of (or acquaintance with) the probable consequences of each member of a set of available options).
We experience our actions as free when we are able to deliberate on the available options and when the choice, made in the light of having anticipated effects, appears to be in our power. I have the power of sending this post or not because neither disability nor constraint nor ignorance nor neurotic compulsion stands in my way of sending it or not sending it.
That does not mean that my action is unmotivated if I do send it. But the fact that it is motivated, and that it is consistent with my character and skills (partly determined by them and perhaps completely determined by them together with environmental influences and the actions of other human beings) does not make it unfree. Nor would it free me from the responsibility for the act and its consequences (so far as a reasonable person could be expected to anticipate them).
People are properly praised or blamed for how their character (which shapes their intentions and therefore their choices) determines their actions, not for the randomness by which their actions are caused. The big exception to this rule is praise or blame for children, whose character is largely unformed and is in the process of being formed; praise and blame in this context is not for character, but it is pedagogical in nature, aimed at the future forming of character.
The ancient Stoics were right at least to this extent: if determinism were true, then that would not eliminate the responsibility that adult rational beings have for their actions. I would add that it might be true, although (for reasons given above) we cannot prove it.