How to Divide a(n Individual) Mind: Ontological Complexity Instead of Mental Monism (for a book symposium on Mark Textor's "Brentano's Mind")

In this paper, I address the issue of how to best account from a philosophical point of view for the diversity of our (synchronic) mental activities. The discussion starts with Mark Textor’s mental monism, defended in his book Brentano’s Mind. According to mental monism, our mental life is constituted by just one simple mental act, in which different sub-acts – e.g. seeing, hearing, and self-consciousness – 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 of mind. 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, for example, 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 the early Brentano by arguing that Brentano imports not conceptual, but 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 Brentano’s adoption of 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.