The chief distinctions between atheism and agnosticism rests on the status of knowledge, or as some philosophers further points out, the moral consequences of belief with knowledge and belief without (e.g. Clifford's Ethics of Belief has a story of the ship sailing to the New World, and Huxley's Agnosticism). The central claim being- regardless of your level of sincerety in what you believe, some which may or may not be self-serving, belief without certainty in knowledge is always immoral, and therefore we cannot be either theists or atheists, as both of these makes a claim on not just one's belief, but a belief that necessarily rests on knowledge. Only with justified belief, or belief with full knowledge, can we make religious statements such as "there is a god", or "there is no god" that are moral, since our belief in god often entail exhortations or imperatives relating to other people. (And if one can always keep one's religious belief completely private, then the moral argument is irrelevant- except for when one is harmed by one's own false beliefs).
There is a related but independent second point here- for some people such as the empircists (like Hume, or Mill), or later the Modernists and Logical Positivists, justified belief is significant not based on moral grounds, but based on the principles in which statements' truth value can be derived. Both theism and atheism suffer the same problem in that statements such as "there is a god", or "there is no god" cannot be verified scientifically. They are in a sense, nonsense, this is another ground for someone to be agnostic.
Let me elaborate the first point a little futher and then move onto the second.
The definition of agnosticism (excerpt):
Agnosticism is the view that the truth value of certain claims—especially claims about the existence or non-existence of any deity, but also other religious and metaphysical claims—is unknown or unknowable... agnosticism is a stance about the similarities or differences between belief and knowledge, rather than about any specific claim or belief.
Stanford dictionary of philosophy makes the following distinctions between agnosticism and atheism (excerpt):
To give a correct and fully general account of the nature of justified belief is difficult and inevitably controversial... nevertheless for the present purpose of distinguishing atheism from agnosticism it is good enough to treat
knowledge as at least justified true belief... Later we shall look at the question of whether we should say that an atheist is someone who claims to know that there is no God or someone who at any rate believes this.
So an atheist is either- someone who claims to know there is no god, or who does not know, but nevertheless believes the non-existence of a god. An agnostic lacks such belief. This is consistent with the definition provided in my previous post.
The above does not seem to be a wholly satisfying or clear enough distinction, but it does make justified beleif, or belief with knowledge, a key criteria that sets agnostics apart from atheists. It continues with an example:
In the light of these considerations let us consider the appropriateness or otherwise of someone (call him ‘Philo’) describing himself as a theist, atheist or agnostic. I would suggest that if Philo estimates the various plausibilities to be such that on the evidence before him the
probability of theism comes out near to one he should describe himself as a theist and if it comes out near zero he should call himself an atheist, and if it comes out somewhere in the middle he should call himself an agnostic. There are no strict rules about this classification because the borderlines are vague....
Here it has been assumed that Philo regards ‘God exists’ (vagueness apart) as an intelligible sentence to which truth or falsity can be ascribed. If he thinks that the conception of deity is so obscure or so permissive that no truth value can be ascribed to ‘God exists’, perhaps he should extend the notion of ‘atheist’ to cover his position also. “Agnostic’ might suggest that there is something to be agnostic about.
(In the above discussion I have used the argument from the fine tuning as an example of something in the grey area between science and metaphysics. There may be other plausible arguments for theism that Philo could consider, and together with further applications of the Bayesian formula the plausibility might be increased in every case. Nevertheless it still might be quite small even in toto. I am assuming that all the arguments are from plausibility considerations and so can reinforce one another. Of course if the arguments fail because of faults in pure logic, then they do not reinforce one another. The conjunction of several logically bad arguments is indeed no better than one logically bad argument.
Even if various philosophers or theologians use the word ‘God’ in different ways are such that their words are quite unintelligible then they can hardly be said to defend theism. As I have suggested, a logical positivist such as the young A. J. Ayer (Ayer 1936) would have at least been less misleading if he called himself an atheist rather than an agnostic. He neither believes nor disbelieves in God, like the agnostic, but he does not think, as I take it that someone who called himself an agnostic would, that God either exists or does not exist but he does not know which.)
Again, the demonstrations by the author above seems unsatisfactory- The only reason I could attribute to this is precisely because natural language is not scientific and not precise enough. A statement such as "There is a god" or a statement such as "There is no god" is a matter of probability when justified belief, aka, knowledge, is involved in the evaluation of such statements- As the author suggests, we ought to apply methods of verification, external to the natural languages we speak, which contrary to natural languages, are scientific in nature, in order to determine the meaning or truths of those statements. There are further problems in applying scientific methods to prove the non-existence of things, which is why statements of existence is either positively proved, or matters of probabily when positive proof is absent.
The external methods of verification mentioned by the author and me, such as
Bayesian/Confirmation theory, or the
Demarcation problem mostly attributed to Positivists such as Ayer, will be extensively discussed in another thread which I intend to follow up in the near future.
As we can see, the discussion of Atheism vs Agnosticism touches upon several philosophical issues. On the one hand, it deals with morality, which is phasing out in philosophical discussions of the modern and postmodern era. On the other hand, it deals with philosophy of language, the way in which we evaluate truth or falsity of statements in natural language, statements which are intrisically very imprecise, ambiguous and by no means suitable for communicating scientific knowledge.
There are however, pragmatic and practical grounds for making a clear distinction between atheism and agnosticism.

Originally Posted by
Lily
This is all very interesting. It would seem to me that for any two people to communicate effectively, they would first have to come to an agreement on a definition of a word. I suppose one could argue all day long about the "common" definition of the word "atheism" vs. the "philosophical" definition, but I'm not sure that argument would get either one any closer to an understanding or respect of the others' true position.
It is important indeed to agree on definitions when communicating. However, the next question would be "who's definition shall we go by?". Well, in philosophical discussions, often there are standardly accepted definitions available on subjects which people have established- precisely because others before us have gone through the same process as we are about to, and we do not need to reinvent the wheel all over again, to try to come up with new definitions from scratch.
However, and without appealing to authority, let us simply ask- why should we use the "common" definition (in Lily's words), if there is one, above the "philosophical" definition? This begs the question- What is the "common" definition if it is not the philosophical one when involved in a philosophical discussion about religion?
I am by no means a Sophist, and I am not interested in vacuous rhetorics. So I will simply say that there is probabily none- unless one strongly believes that one's own personal definition, is conventionally, widly, and popularly accepted by everybody else in the world. When one is not knowledgable about the issue she speaks of, one often believe that to be true, that she really could make a claim on the definition of a pre-existing word with a pre-existing definition because of her own ingenuity, there just might be a chance where she outshines all the thousands of years of her forerunners and overwrites them. Unfortunatly, the self-righteous "common" definition usually don't outshine its forerunners, and have a very short lifespan when serious people are involved, which is why people need to study and have some basic understandings about the topics in which they talk about.
I have so far demonstrated the overlaps of the definitions of the terms involved, but there are certain key basis for their distinction nevertheless- so for example, Antitheism differs from atheism in that it is an opposition to the belief of a god in some social context addressing topics related to organized religion, or opposition of god (questioning/denying the key definitions which constitutes god), whilest atheism is the explicit denial of god's existence, or a belief of his non-existence, both of which runs into the problem that there cannot be positive proofs of non-existence, and therefore atheism is a belief not based on knowledge alone, or certainly not based on knowledge which are verifiable via scientific methods. This epistemological problem is what distinguishes atheism from agnosticism- not because people who claim to be agnostic wish to appear sophiscated- but rather, some makes the distinction based on moral grounds (like Clifford), some held onto their integrity as scientists and modern philosophers- there just couldn't be a justified belief either way.
The way in which we convey these distinctions with our language is also intricate, and worth looking into. I have mentioned in the earlier posts-(denial/believe in the non-exitence), (lack of belief), (against the belief), are three different things. If it is not entirely clear at this point, I will give examples to distinguish them.
Making a statement such as "There is no unicorn in the world" is the denial/ believe in the non-existence of unicorns- an "a-unicornist", if I may exploit our very primitive "common" language for a moment. The prerequisite of this statement being true is that I know/have positive proof that there are no unicorns. Clearly I do not have a justified belief when making such statements. Now if you replace "unicorn" with "god", you have atheism.
"Either there are unicorns/is the unicorn, or there are no unicorns/is no unicorn", is the lack of belief in the existence of unicorns (Margot: Kind of like saying, per Grice, "I don't believe in existence of unicorns, but if there are some unicorns which I am not aware of somewhere, fuck it"). The person making the statement can be said to be an "ag-unicornist". He does not know nor can he pass verdict with his own knowledge, affirmative or otherwise on the subject of the existence of unicorns. Or, suppose someone is not even aware of what a unicorn is (say, he is from the Amazons or Tibet), and hears the above statement (or its negation), he obviously cannot decide either way- but he can still say "either X or Y", or "Not X, or if Y, etc", X being affirmative of the unicorn's existence, and Y being the negation of unicorn's existence. Now if you replace "unicorn" with "god", you have agnosticism.
"I oppose the belief in unicorns/You should never believe in a unicorn", and "I am against unicorns (presupposing that they exist already)", are "anti-unicornist". Against the belief of X, or against X. If you replace "unicorn" with "god", you have antitheism.
Different terms are used differently, in fact, defined differently by philosophers who came to pass, the "philosophical definition" in Lily's words- for precisely the reason that we do not want to confuse ourselves in relevant discussions due to their commonalities. There are certainly overlaps in religious concepts, but when all three (atheism, agnosticism, antitheism), or all two (atheism, agnosticism) are involved in a discussion at the same time, then their overlaps must take a backseat, and their differences be accentuated. This is not because we want to look like smart asses, it is because we want to avoid confusion! The same thing could be said about a discussion about the differences between modern conservativism vs neoliberalism, or communism vs socialism, or catholicism vs protestantism. Nobody wants to argue whether sugar is an ingredient when talking about the differences between coke and pepsi. It would be a futile and stupid effort based on misconceptions of the topic.
I mean, this is why even within agnosticism and atheism, there are
strong vs weak, and
explicit vs implicit. The distinctions become important when certain subjects are invloved, I won't go into the minute details as the links are already provided. The point remains, and I think it should be fairly clear by now, philosophical discussions assume established terms and definition unless you have a reason to ignore them. But then the burden of proof is on the one who ignores the established definitions. To flout them and to expect everybody else to abide by one's own personal definitions, is intentionally misleading if not outright demonstrating one's own ignorance about the subject matter in which she is talking about.
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