The "Big Book" is a thought experiment

Thought experiment

A thought experiment or Gedankenexperiment considers some hypothesis, theory, or principle for the purpose of thinking through its consequences...

 developed by Ludwig Wittgenstein

Ludwig Wittgenstein

Ludwig Josef Johann Wittgenstein was an Austrian philosopher who worked primarily in logic, the philosophy of mathematics, the philosophy of mind, and the philosophy of language. He was professor in philosophy at the University of Cambridge from 1939 until 1947...

 about the nature of ethics

Ethics

Ethics, also known as moral philosophy, is a branch of philosophy that addresses questions about morality—that is, concepts such as good and evil, right and wrong, virtue and vice, justice and crime, etc.Major branches of ethics include:...

 and the verifiability of ethical knowledge

Knowledge

Knowledge is a familiarity with someone or something unknown, which can include information, facts, descriptions, or skills acquired through experience or education. It can refer to the theoretical or practical understanding of a subject...

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Summary of The Experiment

Wittgenstein's thought experiment goes as follows: Suppose there were an omniscient being who knew the position and movements of all of the physical bodies in the world (i.e. all of the natural properties of the world). Suppose further that this omniscient being wrote all of these natural facts in a large book, such that the book contained a thorough and comprehensive description of all of the things in the world. Wittgenstein argues that nothing in this book would even reference the idea of ethics or contain any statement that could logically be construed to imply an ethical judgment. His conclusion is that the concept of a moral fact is nonsensical, since even if one were to know every fact concerning the world that person still would not know anything factual about the realm of ethics.

In Wittgenstein's Words

"No statement of fact can ever be, or imply, a judgment of absolute value. Suppose one of you were an omniscient person and therefore knew all the movements of all the bodies in the world dead or alive and that you also knew all the states of mind of all human beings that ever lived, and suppose you wrote all you knew in a big book, then this book would contain the whole description of the world; and what I want to say is, that this book would contain nothing that we would call an ethical judgment or anything that would logically imply such a judgment."