You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Materialism

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:
Materialism does not define what?

how material substance should be characterized.