1. Freewill.  (See here.) Arguably, if some very quick and intelligent person were around a second after the Big Bang, and knew the position (and charge and mass etc.) of every particle in the universe at that moment, and had the Complete Book of Scientific Laws, then, at that moment, billions of years before you existed, that person could have worked out precisely what you would do at every moment throughout your lifetime. After all, your actions are caused, and their causes are caused, and so on, all the way back to the Big Bang. Such predictability can make it seem as though you could never have done otherwise, that you are at the mercy of the causes of your actions, that you never freely and responsibly do anything.

You might protest that there is randomness in the universe. But would that be reassuring?  Ordinarily, it is precisely when someone acts unpredictably—out of character—that we begin to doubt that they acted freely or responsibly. So the causal ancestry of our actions seems both at odds with and yet necessary for those actions to be free!

It might seem as though what is needed is that our actions be caused by events that cause themselves. But is that coherent? Or perhaps, with Hume, we should settle for the idea that free actions are those caused by our decisions—even if those decisions are caused, and their causes caused, and so on.

2. Induction. Is it reasonable to believe the sun will rise tomorrow? Perhaps it is provided it’s reasonable to believe that the following principle will continue to hold: that the future will (largely) resemble the past. But why do we believe it will continue to hold? Perhaps because it has held in the past. But then it looks as though we are using a principle to justify itself. That can’t be legitimate, can it? Yet it is crucial that we provide some answer to this question if we are to retain the belief that science is reasonable.

3. Scepticism. (See here.) Given that our senses sometimes mislead us as to how things are (indeed, given, more radically, that when dreaming or hallucinating we can fail to realise we are not perceiving the world at all) the question arises how we can know anything about the world on the basis of sense experience, e.g. on the basis of what it seems to us we see and hear and touch?

You might not be worried. After all, barometers too sometimes mislead us, but that does not prevent my reasonably judging that it’s raining by looking at my barometer, provided I have in the past established the general reliability of my barometer by independently checking on the weather to see if it tends to be how my barometer says it is. But that model is not reassuring when it comes to my senses. For I have never, and surely could never, establish the general reliability of my senses in any parallel way. I can’t check independently of my senses that the world tends to be how my senses say it is.

Common sense certainly suggests I know I am sitting at a table; but an account is needed of how I could.

4. Ethics. (See here.)  If it would be wrong to farm and kill humans for food, why isn’t it wrong in the case of animals? The obvious reply is that the welfare and lives of animals matter less than the welfare and lives of humans. But why? If you say “Because animals are not human; they lack our genetic code”, that looks akin to racism or sexism. In itself, a genetic code looks no more morally relevant than skin colour or gender.

Perhaps the point is that humans tend to have (because of their genetic make-up) some other morally relevant feature, e.g. high levels of intelligence, self-consciousness, plans and projects for the future. But, even if such features are morally relevant and tend to be had by adult humans, not all humans have them. Newborn human babies lack them, for example; they are less intelligent than adult pigs, arguably, which tend to be at least as smart as dogs.

You might say, “But newborns have more potential”.  True, and so do foetuses, so if potential is important, this might commit you to opposing abortion.  But it also needs explaining why potential is relevant. And, anyway, profoundly brain-damaged humans arguably lack such potential. So it is a good question whether there are legimate reasons for thinking that pigs' welfare and lives matter less than such humans'. And if there aren’t, and if it would be wrong to kill such humans, it’s worth asking how it could be permissible to kill pigs.

They taste good, certainly, but can that be enough?

5. Personal identity. (See here.) In virtue of what are you now the person in some ten-year-old photo? You’ll have (almost) no cells in common with that person; most of your cells are replaced every seven years or so.

Maybe that should not stop us saying you have the same brain; and perhaps that’s what counts. After all, suppose Osama bin Laden is in a cave in Afghanistan. There is a temptation to say that if Osama’s brain were now transplanted into George Bush’s skull, the resultant person in Crawford, Texas, looking just like Bush, would in fact be Osama.  (After all, he'd have jihadist views, surely, and remember planning the 9/11 attacks.)

But why emphasise the brain rather than some other organ? Presumably because the brain supports mental states. But then perhaps the mental states are really what matters. Hence Osama would end up in Texas even if we only scanned his brain before destroying it, and then imprinted his mental states on Bush’s pre-wiped brain. But what if the process of psychological transfer went wrong and we imprinted Osama’s mental states both on Bush’s brain and on Putin’s? Is Osama now in Texas or Moscow?  He can’t be in two places at once.

One answer is that a person is not a continuant thing, like a table, but an unfolding event, like a football game, and that we should claim that what was in the Afghan cave prior to the psychological transfer was not a whole person but a person stage (akin to the first half of a football game), and that the stage in question was an earlier stage both of the person now unfolding in Texas and of the person now unfolding in Moscow. But although this overcomes some of the difficulties, it generates others.

Hence some will go back to the beginning and say we took a wrong turn, right at the outset, when we tied the identity of persons to psychological states rather than to the identity of the animal—the human being—who has those states.  The debate goes on ...

(If, incidentally, you are put off by the outlandishness of thought experiments about brain transplants and the like, remember their usefulness doesn't rest on their being realistic or probable, but rather on their being vivid test cases for our intuitions and theories.  The reason I care about what to say about brain transplant cases is not that I expect one day to be faced with one, but that what I would say about such a case, were it ever to arise, is one way of finding out what I do and ought to think persons actually are -- right now, as it were.)

6. God. (See here.)  Different people mean different things when they say, “God exists”. Some but not all mean that there exists some non-physical agent who is omnipotent, omniscient, and omni-benevolent, and who created and intervenes in the universe. Why believe in such a thing? Some argue that everything, including the universe, needs a cause, so God must exist as a first cause. But why must there have been a first cause? And, anyway, if everything needs a cause, mustn't God?  Some argue that the fittedness of biological species to their environments is evidence that the universe, or at least its species, were designed. But arguably Darwin’s alternative explanation -- couched in terms of natural selection -- is at least equally satisfying, yet simpler.

Still, some will protest, to seek reasons for believing in God is to miss the point: theism is or ought to be about beliefs held on faith.  Some might go further and deny it's about belief or explanation at all, claiming perhaps that the role of religious language, such as "God exists", is quite different from the role of such scientific claims as "Electrons exist".

Still, if the belief and evidence game is being played, atheists will not only deny there's evidence for God, but insist there's evidence against, namely the existence of suffering. God could not prevent murder, some reply, except by eliminating our freewill. But to the extent that we can make sense of freewill (see issue 1 above), a universe in which everyone is free and never murders is surely conceivable. So if God exists, why didn’t He create that universe? And what about the suffering caused not by human actions but by earthquakes (see here)?

Suffering, some theists reply, is required for the development of virtues such as courage and compassion. But others thinks there are alternative routes to such virtues, or that they are valuable only in a world of suffering. This debate, like the others, goes on ...