We have no real value just because value isn't real, but a property of knowledge.
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While things themselves aren't good or evil, facts serve as the basis of our knowledge of them as good or evil. Inherent significance is an oxymoron, there is only extrinsic meaning,
For most this line of reasoning takes us to utter materialism. Meaning becomes not only extrinsic but also relative. What is factually and demonstrably desirable for me becomes the definition of good. If I have the power to enforce my will I should always do so provided I can avoid negative repercussions. Any harm inflicted upon others is meaningless for those others have no value beyond their usefulness to me. This worldview leads to bondage, suffering and stagnation.
Alternatively, the knowledge of an infinite Creator who formed existence out of nothing and maintains creation leads us to the derivation of something not only functional but also wonderful and elegant. This is the knowledge that allows man to escape from bondage and transform himself into something better.
What would that mean that meaning is relative? That content of concepts are in itself arbitrary? They are so evidently, we can think and conceptualize anything. Or that that its only determined in relation to something else? This is would only be so in a fully empiricist epistemology, the opposite of rationalism. The absence of god does not imply a positive definition of good as useful. Suffering is not a result of evil, but a natural condition, the world itself as indifferent to our purposes is the reason we are bound in chains, and there is only one way of escape, building better chains.
Ah you mean materialism as in hedonism, this again is the problem for empiricists (the don't have a priori and therefore can't have nice things).
And how to be certain in the knowledge of (the) idea of good? You have to deduce it a priori, much like a mathematical proof.
My a priori deduction of good is likely to differ from yours nihilnegativum. My definition of good is that of an infinite Creator. Others will define it as some physical pleasure and fall into the materialism described above. You have conspicuously failed to provide us with your definition.
Most likely, but the reasons used would be subject to critique. Even your definition of good is not the infinite creator, but something else. Leibniz defined this world as the best possible world in order for the world to have a good reason to exist, this was god's will as the good itself, that gives the idea of good as perfection, I use the same without the theological conceptual framework .. my definitions are very technical, I'm afraid it would not go far, explaining them here in more detail. Suffice it to say, that the idea of good is one of perfection, that there is an essential duality to it (the a priori idea of perfection as imperative and a necessary relation of its determinations and a posteriori practical perfection as a contingent relation), that this leads to to the specific practical idea of good for a human society as the identity of the good of individual and the good of common (everyhing done in a way not at the same time beneficial to the individual and the society as a whole is bad), etc. but thats not really the nihilist part, nihilism is about ontology, the lack of onlogical ground, the absence of the infinite that would prevent time from having a reality.