natural scientist
Refers to a scientist in the natural sciences (physics, chemistry, biology, etc.).
He left famous research as a natural scientist.
In old documents, 理学者 can refer to a philosopher.
哲学者 is the modern standard word for 'philosopher', replacing the obsolete sense of 理学者.
Compound of 理学 (natural science) + 者 (person). The obsolete sense 'philosopher' reflects an older, broader use of 理学.