It's not clear what it would mean (or what it would take) to show that there was an effect without a cause (unless we just define an effect as something that is caused, in which case there'd be nothing to show). We certainly have not shown (proven) that every event has a cause in the sense that we have not, and could not, pick out the causes of every event that has ever happened. Rather, we tend to assume that all events have causes, except perhaps those people who assume that free choices are uncaused or who assume that there is a first event that was not caused by any prior event. It's an assumption that tends to work for us--that is, it helps us explain things in science and our everyday life--and it is an assumption that does not have any clear counterexamples (but again, it's not clear what a counterexample would look like). So, as you suggest, this thesis of Universal Causation (UC) might be the most justifiable.

However, one might think that a possible counterexample involves the events described by quantum physics, since it looks like those events are indeterministic: given the exact same prior conditions (or causes) and the same laws of physics, more than one effect might occur. For instance, the electron shot at a barrier might go one way or another and nothing explains (or causes?) which way it goes. However, a better way to describe indeterministic events is that they are probabilistically caused. Quantum physics does not entail randomness--rather it describes objective probabilities between events. So, one can say that the set of events that leads to the electron's hitting the barrier causes it to end up in position 1 (with 50% probability) and causes it to end up in position 2 (with 50% probability). Wherever it ends up, it was caused to end up there by prior effects.

OK, all this is just to set up the take-home message which is that the thesis of UC (Universal Causation), which says that every event--or at least every event after the first event--has a cause, does not entail the thesis of determinism. Determinism is the thesis that: Necessarily, given the same prior events and laws of nature, the same later events occur. UC is consistent with indeterministic causation and hence with the falsity of determinism. So, even if we answer your first two questions 'no', that does not mean we have to answer your third question 'yes'.

Finally, "Hard Determinism" is the thesis in the free will debate that says (a) determinism is incompatible with free will, (b) determinism is true, and therefore (c) free will does not exist. Lots of philosophers (like me) are compatibilists who reject (a) (if interested, see my prior responses to questions in the Freedom category). And most philosophers reject (b) since they accept that the dominant interpretation of quantum physics is indeterministic. But many still think determinism, if true, would rule out free will and do so for reasons that make Universal Causation just as threatening, so indeterminism doesn't help. (Basically, they think that our decisions, like everything else are caused by prior events, ultimately by events over which we have no control, so we do not have ultimate control over our decisions, and they think such ultimate control is required for free will--again, I see no reason to think free will requires such unattainable powers!). These philosophers sometimes call themselves "Hard Incompatibilists" (or Skeptics about free will), and they are the descendants of the Hard Determinists.

This is complicated stuff and I've tried to keep it brief. But I hope this helps!

Just one footnote to Sean. If homosexuality is a choice, it's not, as Richard Mohr once pointed out, like the choice of what sort of ice cream you're going to buy. Here's a thought experiment to try. Think of someone you find sexually attractive. Now try to choose not to have that response. Part two: think of someone you don't find sexually attractive. Now try to choose to be attracted to them. Step three: repeat steps one and two for broad categories of people where you find you have pretty stable patterns of attraction. If you are anything like me, you'll find that the attempt to choose doesn't get you anywhere.

Just how we end up being sexually attracted to the people we're attracted to is not easy to say. What seems pretty clear is that it's not in any ordinary sense a choice,

Of course, having predilections is one thing; that may not be a choice. Acting on them is another; that usually is a choice. If a case could be made that it's wrong for homosexual people to act on their attractions, then the fact that their orientation is not a matter of choice wouldn't simply excuse them. In fact, however, the arguments I've seen are pathetically bad. A bit more carefully, there are of course lots of situations that call for not acting on our attractions. But that said, the idea that there's some special problem about homosexual attraction is a lot harder to defend than some people seem to have thought.

The question of whether homosexuality is a choice may be morally relevant. If, as is commonly--although not necessarily correctly--assumed, agents are only responsible for what they choose or do, then only if homosexuality is a choice can one be responsible for being a homosexual, and consequently, subject to moral or religious sanction for being a homosexual. The question of whether homosexuality is a choice, while a vexed one, remains unsettled, although it does appear that the balance of evidence currently seems to favor the view that homosexuality is not a choice.

Although the question of whether homosexuality is a choice may thus well be taken to have moral significance, and although it has been linked to the issue of the legality of gay marriage, it is not clear to me that the issues are indeed related. The issues might be taken to be related in the following way. If marriage is supposed to reflect the 'natural' suitability of the partners in question, then, if homosexuality were indeed a choice, homosexuality would not be a 'natural' condition, and, consequently, homosexual unions would not be natural, and should not therefore be permitted. (By parity of reasoning, if homosexuality were a natural condition, then homosexuals could marry.) To my mind, however, the conception of the institution of marriage presupposed here is mistaken. The legality of homosexual marriage should, to my mind, turn on only on the question of whether the people in question are qualified to enter into the contract recognized as marriage by the state. Insofar as the parties in question have attained the 'age of consent'--the appropriate age to enter into such a contract--they should have the right to so contract. (By contrast, two underage homosexuals or heterosexuals, or one homosexual or heterosexual of legal age and another who is underage, or a person of legal age, regardless of his or her sexual orientation, and some animal, may not legally enter into such a contract.) Since, to my mind, the question of the permissibility of gay marriage is a question of whether gay people may legally into a state-sanctioned marriage contract, the only relevant question, to my mind, is whether the parties in question are indeed fit to contract. It's irrelevant, on this view, whether the identity of the parties in question is natural or artificial--that is, a matter of artifice, or choice.

Just to add a little to Eddy's fine response, which neatly limns both what position is taken on free will by most philosophers and the general state of play of the debates around free will. I just want to comment briefly on the status of the debate on free will for moral prescriptions--which I take to mean the justifiability of ascriptions of praise, blame, etc. (however they are understood--and there is debate, especially, on how to understand the nature of blame: for a sophisticated, but accessible and very clear treatment of this topic, see T. M. Scanlon, Moral Dimensions: Permissibility, Meaning, Blame). Both compatibilists and incompatibilists agree that ascriptions of praise and blame are justified just in case agents are free, but they differ--as Eddy pointed out--with respect to how they define free will, which definitions reflect differing views on metaphysical and scientific views about the nature of human beings and of the world. Very roughly, incompatibilists believe that in order to be free, agents must have the capacity to originate their own free choices, or, in an alternative formulation, that a free choice is one that is such that an agent can either do or not make the choice (this capacity is known as the 'Principle of Alternative Possibilities', or PAP); compatibilists, by contrast, tend neither to think that freedom requires that agents be the sources of their choices, nor that they have the capacity for alternative possibilities. But these are not the only positions that one might take with respect to the justification of ascriptions of praise and blame. Certain utilitarians, for example, believe that ascriptions of praise and blame are meant simply to encourage certain types of behavior (praiseworthy behavior), and discourage other types of behavior (blameworthy behavior), and think that ascriptions of praise and blame can achieve this end regardless of whether human beings are free. (For a classic statement of this position, see Moritz Schlick, "When is a Man Responsible?," or J. J. C. Smart's paper, "Free Will, Praise and Blame," which is collected in the second edition of Gary Watson's excellent collection, Free Will.) Whereas the sort of position advocated by Schlick or Smart is agnostic on the question of whether agents have free will, in recent years, Derk Pereboom has developed a related position, 'hard incompatibilism', according to which, roughly, because agents do not have free will (understood in an incompatibilist sense, as the capacity for agents to be the sources of their choices or actions), therefore the only use that ascriptions of praise and blame can have is that of encouraging good behavior and discouraging bad behavior: essentially, then, on this view, moral practices become a matter of social control.

Now although most philosophers work their way into the problem of free will by considering whether freedom is compatible with determinism, there's an alternative route, adopted, for example, by T. M. Scanlon in his magisterial work, What We Owe to Each Other: one might begin by getting clear about the nature of morality, and moral obligation, or moral practices, and then and only then turning to the question of what sort of freedom is required to underwrite such practices. The virtue of such an approach, to my mind, is that it keeps very clearly in focus just why it is we care about freedom--because we're concerned about the viability of our moral practices--and then seeks to determine just what sort of conception is necessary for those practices to continue to function as they do. (Such an approach, to my mind, casts serious doubt on the sort of revisionary account of moral practices advocated by Pereboom and philosophers such as Smart and Schlick. But this is a vexed matter.)

There is not a prevailing consensus on the questions of (1) whether free will is compatible with determinism and (2) whether humans have free will. However, I would estimate that close to 2/3 of professional philosophers are compatibilists about free will and determinism (they think determinism poses no threat to free will), with the other 1/3 roughly split between libertarians (who are incompatiblists who believe that we have free will, and hence that determinism is false) and hard incompatiblists or skeptics about free will (who are incompatiblists who believe that we do not have free will because determinism is true and/or indeterminism would not help secure free will). These estimates are based on a large-scale survey I conducted (along with Thomas Nadelhoffer) and on another large-scale survey conducted by David Chalmers and David Bourget (see here; lots of other interesting results too). These survey results may be slightly off for various reasons (e.g., for these estimates, I've removed the 10-15% who pick "other" when offered only the three categories mentioned above, and among philosophers whose research focuses on free will, there may be more incompatiblists).

Since compatibilists believe we have free will (though they need not, and I argue that there are threats to free will distinct from determinism that make "skeptical compatibilism" a live position), and libertarians believe we have free will (though they argue that compatibilist free will is not enough), it looks like about 85% of philosophers believe we have free will, which is almost as many as the 90-95% or so of non-philosophers who, in my surveys, say they believe humans have free will.

However, it may be that many non-philosophers associate free will with a non-physical mind (or soul) and with the power to make choices ungoverned by natural laws (I'm not sure how committed "the folk" actually are to these theoretical claims and am testing that by doing "experimental philosophy" surveys). If the folk understanding of free will does include such commitments, then most philosophers (and probably most psychologists and neuroscientists) would argue that we lack that sort of free will.

So, whether we have free will or not depends on how you define "free will" and it depends on metaphysical and scientific facts about the way humans and the world work. For a good introduction to these debates, you may want to look at Robert Kane's A Contemporary Introduction to Free Will. And you can look at some of the answers panelists have given here at AskPhilosophers to questions about freedom.

Given your question, you may be interested in a discussion of Strawson's NYTimes article at the free will/moral responsibility blog, Flickers of Freedom, here.

There's also a discussion on retribution and punishment (and psychopaths) at the blog here.

You'll see in these discussions that there are plenty of philosophers (called compatibilists) who think that free will and moral responsibility are possible even if determinism is true, and who reject Strawson's argument against the possibility of freedom and responsibility. These compatibilists will generally say that retributive punishment is justified, though they might also think that punishing (or treating) criminals for consequentialist reasons (such as deterrence and rehabilitation) is also important.

My own view is that we can have free will and moral responsibility (determinism is irrelevant to this issue), but that we have less than we think (because the sciences of the mind are showing that we have less self-knowledge and conscious control than we think). So, I think retributive punishment can be justified, but usually criminals deserve less of this sort of punishment than our system doles out. We should put more emphasis on the forward-looking purposes of punishment (or, if you wish, call it quarantine and rehabilitation).

Given the premises, I can't think of anything but a consequentialist defense of punishment or "correction." I also believe that that some of the arguments around this issue provide an opportunity for reflection on our powerful attachment to the rhetoric of "taking responsibility."We do not hear enough about our responsibility as a nation to create communities that nurture a sense of morality and connection with others. Many only want to talk about personal responsibility and it is often with a punitive edge. There might be other terms in which to couch the issue of moral striving.

Here's one answer I have given to a related question.

A large number of philosophers believe that one may affirm both determinism and freedom of the will. Probably at least half the panel does, though I do not. I suggest that a person does an act freely if she does the act and has the power to do otherwise (all other things remaining the same). And if determinism is true (determinism is, roughly, the idea that every event is necessary given antecedent and simultaneous events and the laws of nature) then persons lack the power to do any act other than the act they do. In any case, for most philosophers who affirm that we are free, we rarely think that freedom is without any context or absolute. In other words, if you are free, you are free to do some specific act or adopt some particular practice, and so on. You righly note the influence of family and other factors in shaping our thinking and action as adults. And even now, as adults, we may be under the influence of all sorts of forces and conditions. Nonetheless when you do reflect freely about (to use your example) the ethics of abortion, this would be a case of when you have the power to weigh reasons pro and con and come to your own conclusion.

This is an important question, since it might be that one of the reasons we worry about whether we have free will is that free will is required for life to be meaningful. If so, then any threat to our free will would also make life meaningless. (Actually, as I write that sentence, it makes me wonder if a person's life can only be meaningless, in the ordinary sense of that word, if it has a possibility of being meaningful--is a worm's life meaningless or does that word simply not apply?) But is free will required for life to have meaning?

As usual (with philosophical questions like this), a lot depends on what we mean by 'free will' and 'meaningful life'. My own view is that a theory of free will needs to be about the powers of control that matter to us, so it doesn't make sense to define free will in such a way that losing it would not matter and such that having it would not matter. If, for instance, free will is defined as some magical ability to exist outside of the natural order of things, then I'm not sure why it would matter if we don't have it. But if, as you suggest, free will is the power to make choices and have your decisions make a difference to what happens, rather than just being a helpless bystander observing what happens, then it would be terrible not to have free will. And I find it hard to see how life could be meaningful without such free will, since it seems like part of what makes life meaningful is deciding what sorts of goals and plans you have and making choices that help you achieve those goals and plans.

I should emphasize that I am a compatibilist about free will and determinism (as I explain here), so I don't think that the truth of determinism would make life lack meaning because it would not make us mere observers whose decisions didn't make a difference. But figuring out how free will works is no easy task. And figuring out what makes life meaningful is even harder.