A scientist is, generally, a person devoted to and producing results in science using the scientific method.
Some people (including many mathematicians) do not classify mathematicians as scientists since mathematicians generally do not use scientific method in mathematical discovery, and scientific method is not valid for proving mathematical results. Nevertheless, due to the historical intimate relationship between mathematics and applications (i.e. the empirical sciences), mathematics is typically listed alongside the empirical sciences, and mathematicians are for that reason often classified as "scientists". Indeed, the distinction between scientists and mathematicians was not common until the 20th century.
