More heat is generated by the words in this lesson than by any argument in the course. Almost all of it comes from three confusions, and once they are cleared the remaining disagreements are substantive and much smaller.
Two things "atheist" means
Consider two people. The first says: there is no God. The second says: I have no belief that there is a God, and I do not claim to know there is not.
These are different mental states and they carry different burdens. The first is a claim about the world and needs defending like any other. The second is a report of where someone's credence sits and needs no defence of the same kind, though it can still be criticised as unreasonable given the evidence.
The literature usually calls the first positive or strong atheism and the second negative or weak atheism, and the everyday word covers both. That would be harmless except that arguments are constantly conducted with one party using one sense and the other party attacking the other.
Notice how lesson five makes the distinction precise, and better than the labels do. Belief is not a switch, it is a credence. The positive atheist has a credence near 0. The negative atheist has one that is merely low, or has declined to form one. The theist has one that is high. There is no sharp line, and the ordinary vocabulary pretends there is.
Example. A theist says: "Atheism is a belief system too, and it takes just as much faith, because you cannot prove God does not exist." Untangle it.
Three separate errors. First, it trades on the ambiguity: negative atheism is the absence of a belief, and an absence is not a belief system, though positive atheism is a position and the charge partly lands there. Second, "you cannot prove" invokes the certainty standard that lesson one disposed of: almost nothing is provable in that sense, including the existence of other people, and holding a well-supported low credence is not faith. Third, it assumes that two positions equally short of proof are equally supported, which is the flattening from lesson one again and is exactly what lesson five exists to refute. What survives is a fair point worth conceding: someone who asserts confidently that no god of any description exists has taken on a burden, and should be asked to carry it.
Now you. A negative atheist says they need no arguments at all, since they merely lack a belief and the burden is entirely on the theist. Is that right?
Answer
Partly, and the unqualified version is too comfortable. It is right that someone who makes no claim is not obliged to defend a claim, and the theist who asserts something does owe support for it. But lacking a belief is not automatically reasonable: a person who lacked belief in evolution while acknowledging the evidence would be criticisable, not because they had asserted anything but because their credence does not match what they have. So the negative atheist escapes the burden of proof and not the burden of proportion. In practice most articulate atheists hold more than bare absence, since they think the arguments of lesson eleven have force, and that is a positive claim which does carry a burden. Retreating to bare absence when challenged and advancing to evidential claims when comfortable is the move to avoid.
Agnosticism is a claim about evidence
The second confusion is treating agnosticism as a refusal to answer, or as a polite midpoint between two rude extremes.
T. H. Huxley coined the word in 1869, and he meant something specific and demanding. Agnosticism for Huxley was a principle of method: do not profess certainty in conclusions that are not demonstrated or demonstrable. It is the application of a standard, not an avoidance of one.
So agnosticism is itself a claim, namely that the evidence does not settle the question, and that claim can be right or wrong. Someone who says the evidence is balanced is asserting something about the arguments of lessons ten and eleven, and can be shown to have misjudged them.
This also explains why agnosticism and atheism are not alternatives on a single line. They answer different questions. Atheism is about what you believe; agnosticism is about what you claim to know. A person can be an agnostic atheist, holding no belief in a god while claiming no knowledge either way, and that is the commonest position among nonbelievers who have thought about it. Agnostic theism is likewise available and has a long religious tradition behind it.
Burden of proof
The third confusion. "You cannot prove a negative" is false, and it is a pity it is so often said, because a real principle sits underneath.
Negatives are provable all the time: there is no prime number between 24 and 28, there is no elephant in this room. What is hard is proving a negative about something whose absence would leave no trace, and that is the actual asymmetry.
Bertrand Russell made the point in 1952 with a china teapot orbiting the sun between Earth and Mars, too small for any telescope to detect. Nobody could disprove it. The insistence that it must therefore be taken seriously would be absurd, and the reason is lesson nine's: the hypothesis was constructed to be evidentially inert. Its unfalsifiability earns it nothing.
The precise statement is that the burden falls on whoever proposes an entity, and that a hypothesis designed to be undetectable cannot draw support from its own undetectability. This cuts against careless atheism too, since "there is definitely nothing beyond the physical" is also a claim about a vast domain.
Example. Is the God hypothesis relevantly like Russell's teapot?
In one respect and not in another, and the honest answer says both. It is like the teapot in that a sufficiently inscrutable deity is undetectable by construction, and lesson nine showed that such a hypothesis cannot draw support from evidence. It is unlike the teapot in that nobody has ever offered an argument for the teapot, whereas there are arguments for theism with long histories and serious defenders, and lesson ten set out five of them. The teapot analogy is therefore a good answer to "you cannot disprove it, so you should be neutral" and a poor answer to the contingency argument, which does not rest on undetectability at all. Using it against the second is the mirror of the theist using inscrutability in lesson eleven and plausibility in lesson ten: a move that works in one place deployed where it does not fit.
Now you. Why is "extraordinary claims require extraordinary evidence" a consequence of lesson five rather than a separate principle?
Answer
Because it is just Bayes with the prior made visible. An extraordinary claim is one with a low prior, and to reach a high posterior from a low prior the likelihood ratio has to be correspondingly large, which is what "extraordinary evidence" means. The medical test example in lesson five is the same structure with numbers attached: a very low base rate swallowed a 99 percent accurate test. Seeing that the slogan is a theorem rather than a rhetorical demand matters for two reasons. It shows the principle is not special pleading invented for religious claims, since it applies identically to a new particle or an unexpected medical result. And it shows where a theist can legitimately push back, by arguing that the prior is not as low as the atheist assumes, which is an argument about priors rather than a rejection of the principle.
The positive stance
None of this says what to do on a Sunday, and secular humanism is the attempt to answer that without a deity: an ethics grounded in human welfare and reason, a commitment to inquiry, and the view that meaning is made rather than received.
The standard objection is old and deserves a real answer. Without God, it is said, there is no foundation for morality, so the atheist has no right to call anything wrong.
The ethics course sets out the reply in full, and the compressed version is Plato's. In the Euthyphro, Socrates asks whether the pious is loved by the gods because it is pious, or pious because the gods love it. Both horns hurt the objection. If God commands the good because it is good, then goodness is independent of God and the atheist can have it. If things are good merely because God commands them, then goodness is arbitrary, and had the command been to torture children that would have been good, which almost every believer denies.
The upshot is not that theism cannot ground ethics, since sophisticated answers exist, most of which identify God with the good rather than treating goodness as a command. It is that the simple version of the objection fails, and that moral realism without God is a live position defended by many philosophers who are not atheists.
Meaning is the other half. The humanist claim is that a meaningful life consists in relationships, work, understanding and care, and that these are not made hollow by being finite. Whether that is satisfying is not a question a course can settle. Whether it is coherent is, and it is.
Example. Someone says the low rate of theism among professional philosophers shows the arguments for God fail. What is wrong with the inference, and what does survive?
It is an appeal to authority, and lesson one already flagged the shape when reporting the same kind of number about scepticism: a majority of experts is not an argument, and this majority could be explained by selection into the profession or by shared assumptions rather than by the arguments. What survives is still worth having. Expert opinion is evidence about where the arguments lead, weaker than the arguments themselves but not nothing, in the way that a surgeon's judgement is evidence without being a proof. The right use is as a check on your own reading: if you find an argument in lesson ten overwhelming and most specialists do not, that is a reason to reread rather than to conclude they are fools.
Now you. Which of the positions in this lesson is available to someone persuaded by the contingency argument of lesson ten but also by the evidential problem of evil in lesson eleven?
Answer
Several, and the most natural is a deism: accept a necessary being as the explanation of why anything exists, and deny that this being is the perfectly good, intervening God the problem of evil targets. That combination is coherent precisely because, as lesson eleven noted, the two arguments aim at different targets. Another option is theism plus sceptical theism, holding that the evidential argument fails because we cannot judge divine reasons, at the cost lesson nine identified. A third is atheism plus a brute fact, denying the Principle of Sufficient Reason and accepting that the universe exists with no explanation. All three are defensible and each pays somewhere. Being able to see that the two lessons do not simply cancel out, and to name the positions that survive both, is most of what lesson fifteen asks for.