How are we to understand dementia? The main argument involves an analysis (in Chapter 2) of intentional mental states, using Wittgenstein's discussion of rule-following, which suggests that such states demonstrate an irreducible, transcendental normativity. This externalist account of intentional mental states highlights the worldly embedding of practices. In Chapters 3,4 and 5, this analysis is applied respectively to the disease, cognitive neuropsychology and social constructionist models of dementia. Whilst clinically and scientifically useful, none generates an adequate account of normativity. The (...) Wittgensteinian analysis supplies a constitutive (as opposed to causal) account that supports the notion of dementia-in-the-world (Chapter 6). A full understanding of dementia requires the human-person-perspective in order to accommodate all that dementia amounts to in the normatively-constrained world. The sub-plot considers our understanding of the person. Rather than the Locke-Parfit view, which stresses psychological continuity, the Wittgensteinian analysis supports the situated-embodied-agent view of the person (Chapters I and 6). This view and the notion of the human-person-perspective are mutually supportive, so that main and subplot both encourage a broader understanding. The works of Wittgenstein have acted as a primary source, with secondary literature commenting on his works. In discussing the models of dementia, I have cited primary sources. I have also considered philosophical works pertinent to the particular models, usually in connection with the mind-brain problem. The thesis concludes that there is no single way to understand dementia, but any understanding will be from the human-person-perspective, in accord with the situated-embodied-agent view and reflecting an externalist construal of intentional psychological states. This has implications for further research in philosophy, medical ethics and gerontology. The unique application of the Wittgensteinian philosophical analysis to clinical reality suggests an approach to people with dementia that stresses personhood in the context of embedded, embodied histories and continuing relationships with others. (shrink)