Is nihilism the logical extreme of atheism?

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All-knowing oracle? No, but there is this thing called "experience" in which "language" is used to convey "objects in experience" through the use of "concepts" by which we determine their "validity" as "truth statements" about the world via analyzing their "definitions," internal consistency, and correlation to objects in said experience. Not rocket science. Just basic philosophy.

Experience? Oh, you mean that subjective thing that is different for everybody and which changes over the course of everyone's life? You don't know the truth yet, then... as you've not experienced all. Wait a few years, it'll surely come to you... just like it has come to everyone else

Based on experience, theism is equally as sound an explanation as atheism... if not a better one.

There's a reason you call it "basic" philosophy.