Context: Materialism is a form of philosophical monism which holds that matter is the fundamental substance in nature, and that all phenomena, including mental phenomena and consciousness, are identical with material interactions.

Question: What is materialism?
Answer: a form of philosophical monism

Question: In nature, this is an essential substance?
Answer: matter

Question: What is not a fundamental substance in nature? 
Answer: Unanswerable

Question: Mental phenomena and what else are not identical with material interactions?
Answer: Unanswerable

Question: What is the definition of philosophical monism?
Answer: Unanswerable