Art historian: This painting, purportedly by Mary Cassatt, is a forgery. Although the canvas and other materials are consistent with most of Cassatt' s work, and the subject matter is similar to that of Cassatt' s finest paintings, the brush style of this painting is not found in any work known to be Cassatt' s. Hence this painting is definitely not a genuine Cassatt.
The art historian's argument depends on assuming that the most characteristic feature of Cassatt's work is her brush style.