A person may lack capacity due to a disability such as intellectual disability, dementia, brain injury or mental illness.
A person may lack capacity temporarily, for example during a coma following an accident.
A person may be able to make some decisions but not others.
An example case study
Tom has an intellectual disability. When he develops a toothache he goes to the dentist who explains to Tom and his father that his tooth is decayed and needs to be removed. With the support of the dentist and his father, Tom is able to understand what the proposed treatment will mean for him and provide his own consent to the treatment. However when Tom's dentist recommends a general anaesthetic to remove a number of teeth, Tom is not able to understand the risks of the anaesthesia. Tom's dentist assesses that Tom is not able to give consent for the treatment and talks to Tom's
person responsible instead.