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The Latin term compos mentis translates as “of composed mind,” referencing the idea that a person has intact memories, understanding, and decision-making capacity. People may have mental illness and still be considered compos mentis, as long as the condition does not interfere with the ability to understand communications and be aware of the potential consequences of various decisions. People cannot enter into legal contracts if they are not of sound mind, as this could be considered exploitative.
Adults are generally assumed to be compos mentis unless information is provided to prove otherwise. They have the capacity to read and understand legal contracts, including the ability to ask questions about aspects of the contract they do not understand. They can make decisions on their own behalf, in addition to representing the interests of dependents like children who are not considered capable of making legal decisions.
In a health care setting, individual patients must be evaluated to see if they can be considered compos mentis while they are making decisions about their treatment. Patients in a profoundly altered mental state due to brain injuries or drug use, for instance, may not be able to make sound medical decisions. The hospital can compel them to receive treatment or request that a guardian be appointed to make medical decisions until the patient has recovered enough to start making choices.
When someone is non compos mentis, lacking the capacity to make decisions, that individual cannot be held to contracts and other decisions made. In these cases, people may not have understood what they were agreeing to and were probably not familiar with their legal rights to refuse, request alternatives, or get information about unclear aspects of a decision. In addition, they may not have understood the risks and benefits of different decisions and couldn't comprehend the consequences; someone agreeing to sign away a home, for example, might not understand that this means the home will be occupied by someone else.
If there is doubt about a person's mental capacity, specialists like neurologists and psychologists can be called in to examine the person and provide advice. These specialists may agree that a patient does not have the capacity to make decisions, although previously expressed wishes should be considered, if they are available. In other cases, they may determine that this capacity is present, but that special care should be taken to make sure the patient fully understands the available choices.