Passage Summary: An art historian claims a painting is fake because its style doesn't match any of the artist's other known works, even though the materials are correct.

Conclusion: The painting in question is definitely a forgery and not a genuine work by Mary Cassatt.

Reasoning: While the materials and subject matter are consistent with Cassatt's work, the brush style used in this painting is not found in any of her known paintings.

Analysis: The argument relies on a significant logical gap between 'known' works and 'all' works. The historian assumes that because this specific brush style hasn't been seen in Cassatt's *known* portfolio, she could not have possibly used it in *any* painting she ever created. To make this argument hold water, we must assume that the known works are a complete or perfectly representative sample of her entire output. Look for an assumption that bridges the gap between the absence of a feature in known examples and the impossibility of that feature existing in a genuine work.