noun
mental capacity
Legal term referring to a person's ability to understand and make decisions, especially regarding contracts and legal acts.
意思能力がないと判断された場合、契約は無効になります。
If a person is judged to lack mental capacity, the contract becomes void.
行為能力 (capacity to act) is a broader legal concept that includes the ability to perform valid legal acts, while 意思能力 specifically refers to the mental capacity to understand and form intent.
Compound of 意思 (intention, will) and 能力 (ability, capacity). A modern legal term, likely coined in the context of civil law.