Spinoza’s philosophy of mind has been subject to widely divergent interpretations. What explains this lack of consensus? The principal reason is that Spinoza’s notion of an attribute and its relation to his substance monism is poorly understood. This chapter begins by setting out some interpretative difficulties regarding Spinoza’s notion of an attribute in general. It will then explain Spinoza’s conception of the attributes of thought and extension in particular. Next, it will explain how Spinoza argues for the structural similarity of the mental and physical realms from his claim that the mind and the body are one and the same thing conceived under different attributes. This will require developing a new interpretation of Spinoza’s notion of attribute. This interpretation will both explain why his philosophy of mind has been subject to such contradictory interpretations as well as solve a host of interpretative difficulties that have long vexed commentators. The chapter will conclude by explaining how Spinoza’s denial of mind-body causal explanation is compatible with his assertion of mind-body identity.