Assessments of mental capacity in England and Wales are governed by the Mental Capacity Act (2005), which came into effect in 2007. The MCA built upon, but also largely superseded, a rich earlier history of case law addressing mental capacity. However, the underlying issues raised in this earlier history are still relevant to present-day practice.
The Eastman Test
In one celebrated case prior to the MCA, an expert witness proposed what came to be known as “the Eastman Test” for the assessment of decision-making capacity. The Eastman Test resolved the decision-making process into three stages:
[T]here are three stages to the decision: (1) to take in and retain treatment information, (2) to believe it, and (3) to weigh that information, balancing risks and needs.
Re C (An Adult: Refusal of Treatment)  2 FCR 151
The Eastman Test forms part of the pre-history of the MCA definition of mental incapacity, but unlike the Eastman Test, the MCA test does not make any mention of belief of treatment information.
The MM Dicta
In one of the last court rulings on capacity prior to the coming-into-force of the MCA, Mr Justice Munby (as he then was) explicitly addressed the relation between believing information and capacity as defined by the MCA and its relation to the ability to understand and the ability to use, or weigh it:
If one does not ‘believe’ a particular piece of information then one does not, in truth, ‘comprehend’ or ‘understand’ it, nor can it be said that one is able to ‘use’ or ‘weigh’ it. In other words, the specific requirement of belief is subsumed in the more general requirements of understanding and of ability to use and weigh information.
Munby’s ruling in the MM case has exerted a continuing influence in case law. For example, one 2019 ruling explicitly invoked Munby’s obiter, concluding that:
The case law makes it clear that a failure to believe is a failure to understand and use or weigh in the context of the specific decision-making exercise engaged
The general philosophical propositions upon which Munby’s obiter seems to rely are open to challenge. It is in fact very common to understand information that one does not believe. (Think of the circumstance of a fact-checker identifying false statements in a politician’s speech. The fact-checker understands but does not believe the information provided by the politician.) Munby’s Dicta also rely on the claim that information cannot be used or weighed without being believed. This principle has intuitive appeal but is also open to challenge.
A 2021 ruling in the UK Supreme Court also engaged with Munby’s judgment in MM, but did not address the issues raised by Munby’s obiter.
A 2023 ruling in the Court of Protection once again invoked the MM Dicta, this time in the case of a 19-year-old woman facing a decision about whether to adopt a programme of palliative medicine in the face of her degenerative mitochondrial disease. Invoking the precedent in the MM ruling, the judge ruled that the young woman, known in court papers as ST,
is unable to make a decision for herself in relation to her future medical treatment, including the proposed move to palliative care, because she does not believe the information she has been given by her doctors.
A NHS Trust and ST  EWCOP 40, para. 93