Final answer:
The scientist was describing life forms based on their composition of physical substances such as elements and compounds.
Step-by-step explanation:
When the scientist singled out forms of life based on matter, he was likely referring to life forms that are composed of physical substances. This idea aligns with the fundamental understanding that life, at its core, consists of matter, which encompasses substances that have mass and occupy space. Matter makes up the elements and compounds that form the chemical and molecular basis of organisms. Life forms can thus be described in terms of their material composition, which includes elements such as carbon and compounds like carbohydrates which are integral to their structure and function. The categorization of life forms based on their material composition helps in distinguishing living organisms from inorganic matter and in understanding the biochemical processes essential for life.