Plato explains that in the Myth of Er, Er gives an account of his experience in the afterlife. This story correlates to a complex argument about the relationship between free will and determinism (fate). Er describes two openings in the earth side by side, and two corresponding openings in the sky above. A set of judges directs those souls who are deemed moral towards one of the openings in the sky, and those deemed to be immoral towards one of the openings in the earth. From each remaining opening emerge either souls caked in grime and dust (from the earth) or clean souls (from the sky). These souls are returning from a long journey and meeting again in this middle place of judgment. Following this is a journey to another place in which each soul (having emerged from either opening) is given a choice by Lady Lachesis, daughter of Necessity and who represents the past, as to what kind of life they wish to lead next. Every kind of human and animal life is offered, including every possible combination of wealth, poverty, sickness, health etc.
Er goes onto explain that those who came out of the heavens demonstrated a general trend to not make careful and considered decisions and thereby more often than not chose non-virtuous lives. It is important to note that this is proposed as a trend and not as a law of nature. The souls who hurry their decisions may just as likely make a good choice rather than a bad choice. Here a difference between moral behavior based on habituation and moral behavior based on philosophy becomes distinct. Those souls who emerged from the earth had a deeper understanding of suffering and so were able to cope with difficult decisions and situations. A soul is therefore conditioned by experience. However, there is an eternal regress argument here as it is the moral or immoral actions undertaken in life that determine the judgment passed on that soul in the afterlife, determining that souls experience in the afterlife and there-by the conditioning and influence over future choices.
Here we can see that human choices can at once be conditioned and free. The presence of a sequence of metaphysical causes is expressed as a cycle of reincarnation. This cycle is eternal and so plays the role of having a broad influence over life. It could be described as a set of constraints. In this system, our actions are our own free choice, but the consequences of those actions fit into a causal framework. This is not a strong view of free will as it seems that any individuals actions may at once be free, and also a fated reaction to the actions of another (spurred into being via a cause).
What is possible in the future remains indefinite and the truth or falsity of a possibility relies on the consequence of the inclination of our free will (which is why a representative of both necessity and the past presides over choice). Therefore there are an infinite number of possible futures. The emphasis on choice in spite of habitation and conditioning tells that people are responsible for their lives, albeit this is a weak view of freewill. In other words, man is susceptible to conditioning, though through philosophical rigor may take full moral responsibility for the life he leads.