This paper addresses the issue of how to best account for the diversity of our (synchronic) mental activities. The discussion starts with Mark Textor’s mental monism. According to mental monism, our mental life is constituted by just one simple mental act, in which different sub-acts can be conceptually distinguished. Textor grounds this view in the work of the early Brentano and contrasts it with the theory of the later Brentano, who introduces a mental substance into his philosophy. According to Textor, (...) Brentano needs a substance because he is unable to explain how mental monism can account for the separability of our mental activities, e.g. the fact that I can stop hearing F while still seeing blue. Textor argues, however, that mental monism can solve this problem. I address two issues regarding Textor’s view. First, I challenge his interpretation of Brentano by arguing that the early Brentano imports ontological complexity into our mental life; I defend Brentano’s view against possible criticisms, and I address some objections to mental monism. Second, I oppose Textor’s narrative about the mental substance. I argue that Brentano needs a substance not to explain separability, but rather to individuate our mental acts. I still argue, however, that Brentano’s earlier view (understood in my sense) is better than the substance account for dividing the mind. (shrink)