在社會生活，每一個社會皆會有一些法律規範來確認哪些社會成員可以參與社會中之事務。意思能力（mental capacity）與行為能力（legal competence），是法律行為生效之必要條件。於2009年11月23日，台灣新修訂之成人監護制度正式生效，然而其中很重要之精神疾病患者意思能力之評估，台灣文獻向來並沒有深入之探討。本文回顧國外文獻，將意思能力之評估分成原因成分（疾病等失能成因）及功能成分（實際認知、情緒及行為等表現），著重探討評估功能成分時所需考慮之內容，包括財務處理、個人健康照顧、獨立生活、人際關係與交通等，並且論述選定能力標準時所隱含尊重自主或保護利益之價值判斷，最後，並期許將來法學界與精神醫學界合作進行實證研究，以加強意思能力判斷之科學性與法安定性。

Nearly every society maintains legal norms that define those members of society qualified to participate in social affairs. Mental capacity and legal competence are deemed necessary conditions for legal actions to have legal validity. On Nov. 23, 2009, newly revised adult guardianship provisions came into effect in Taiwan. However, there has been lack of discussion with regard to how assessments of mental capacity and legal competence should be conducted on psychiatric patients. This paper reviewed relevant overseas literature on this subject and followed common practice in separating legal mental capacity into causal and functional components. The causal component predicates the diseases and illnesses that render the disability, while the functional component represents legally substantial
impairments in terms of cognition, emotion and behavior. The paper explored functional component contents, including finance management, individual health care, independence in daily life, interpersonal
relationships and communing. Findings pointed out that in setting up competence standards, a trade-off between respect for autonomy and beneficence is unavoidable. As Taiwan does not have rich empirical data on competence assessments and decisions, collaboration between the legal and psychiatric professions is recommended to engage in relevant research to enhance legal consistencies and the science of competence assessment.