The current mental health legislation continues to be criticised due to the lack of relevant decision-making criterion, such as the use of capacity assessments in regards to compulsory treatment of persons with mental illness and the continued discriminatory nature of the legal frameworks towards those with mental illness. This has lead to increased attention from the proposals of those wishing to reform the law. One such approach that has gained momentum is that of the fusion hypothesis. A fusionist approach to mental health law reform would signal the start of a major paradigm shift in the way in which patients with mental illnesses were to be treated. Fusionist suggestions offers to enhance patient autonomy and eradicate discrimination by introducing capacity-based thresholds into a single piece of legislation allowing both persons with mental illness and those with physical illness to be treated under the same legal framework. This approach would therefore combine mental health legislation with mental capacity legislation to create a unified decision-making legal regime. Fusion offers a patient with mental illness the opportunity to decide for themselves how to be treated using non-discriminatory mental health law and could be particularly attractive to patients, who are liable to be detained under the current mental health legislation, as this approach would seek to give them decision-making rights which they currently do not have. The fusion hypothesis is therefore in need of further investigation; focussing on its claims that it will indeed enhance autonomy and reduce discrimination. To do this capacity relevant legislation will be assessed and its links to autonomy will be discovered. It will be shown that the relationship between these two concepts are not as straightforward as fusionists purport. Following this it will be established that when fusionists talk of non-discrimination what they are really arguing for is 'formal equality' and it will be established that in certain circumstances it may be legitimate to continue with differential treatment of even those with capacity. The thesis will conclude with an examination of the impact of the United Nations Convention on the Rights of Persons with Disabilities providing the fusion hypothesis with a caution as it moves forward.