This book offers a powerful new reading of Spinoza's philosophy of mind, the aspect of Spinoza's thought often regarded as the most profound and perplexing. Michael Della Rocca argues that interpreters of Spinoza's philosophy of mind have not paid sufficient attention to his causal barrier between the mental and the physical. The first half of the book shows how this barrier generates Spinoza's strong requirements for having an idea about an object.

The second half of the book explains how this causal separation underlies Spinoza's intriguing argument for mind-body identity. Della Rocca concludes his analysis by solving the famous problem of whether for Spinoza the distinction between attributes is real or somehow merely subjective.