It depends which dictionary you read. The one that says atheism is the disbelief in God has made atheism redundant with agnosticism. The one that says that atheism is the denial of God has allowed both terms to have unique meanings. If two terms mean the same thing then one is unnecessary. That is not to say that humans don't have terms that mean the same thing, it just means that it makes issues more confusing to do so. This is not a technical argument it is a logical efficiency claim.
Deism is not interchangeable with theism. Deism denies a personal God and theism mandates one that is personal. They cannot be used interchangeably.
Maybe I should have stated this as the way things should be, not what is true of the mess man has made of them.
But atheism is not a mere lack of belief in something. If atheism is just the lack of god-belief, then tables and chairs are atheists. For they lack god-belief. Am I being uncharitable? Suppose someone defines atheism more carefully as lack of god-belief in beings capable of having beliefs. That is still unacceptable. Consider a child who lacks both god-belief and god-disbelief. If lacking god-belief makes him an atheist, then lacking god-disbelief makes him a theist. So he is both, which is absurd.
Obviously, atheism is not a mere lack of belief, but a definite belief, namely, the belief that the world is godless.
Maverick Philosopher: A Bad Reason for Thinking that Atheism is not a Religion
I am certainly not claiming that they have "the same meaning". My claim was merely that they overlap in meaning. The two terms, atheism and agnosticism, according to Merriam Websters still have different meanings. The overlapping, which in no way makes the either of the terms unnecessary, means that one term, agnosticism, can be included in the other term, atheism. But that doesn't mean that they have the same definition. This is the same with Deism and Theism. All Deists are Theists by definition because they hold a belief in the existence of God. I fail to see the problem, so can you illustrate what issue you see more specifically without the erroneous claim that the two terms have the same meaning?
Atheism = lack of belief in the existence of God.
Agnosticism = undecided on whether God does or doesn't exist and, sometimes, that knowledge of this kind is unattainable. (my only point is that Agnostics clearly "lack a belief in God", but the term Agnosticism still certainly does not have the same definition as Atheism; thus, I feel like your claim that one of the terms is redundant is unfounded)
Further, you put words in my mouth in that you claim that I said that Deism and Theism were interchangeable. I certainly did not suggest this. I said, like agnosticism and atheism, Deism is, by definition, a subcategory of theism. Theism, in its general form, simply means "belief in the existence of God". Deism, on the other hand, refers, as you stated, to belief in a non-personal God who does not interfere with our reality. Deists, thus, believe in the existence of God, so they can accurately be considered as a subcategory of Theism. Thus, they are not interchangeable. One term merely includes the other. I not only fail to see an issue with this, but I consider it logically necessary for general terms like "atheism" and "theism" to have subcategories, more specific in nautre, like "agnosticism" and "deism".
I understand that you are using different definitions of the terms in question. I also think that is perfectly fine, as long as that is made clear. But, my point is that your issue with using the definitions found in nearly all reputable dictionaries seems to be unfounded.
