Context: The materialist view is perhaps best understood in its opposition to the doctrines of immaterial substance applied to the mind historically, famously by René Descartes. However, by itself materialism says nothing about how material substance should be characterized. In practice, it is frequently assimilated to one variety of physicalism or another.

Question: Was René Descartes an idealist or a materialist?
Answer: materialist

Question: Materialism does not define what?
Answer: how material substance should be characterized.

Question: Why was René Descartes declared a materialist? 
Answer: Unanswerable

Question: What does materialism say about how substance should be characterized?
Answer: Unanswerable

Question: Materialism defines what?
Answer: Unanswerable

Question: Materialism is not frequently assimilated to what? 
Answer: Unanswerable