What is mental capacity?
Mental capacity is the ability to understand, retain, weigh up and communicate a decision at the time it needs to be made. Under the Mental Capacity Act 2005 capacity is presumed, assessed decision by decision, and losing it for one decision does not mean losing it for all.
The Act sets five principles: presume capacity; support the person to decide before concluding they cannot; an unwise decision is not itself evidence of incapacity; act in their best interests; and choose the least restrictive option.
Capacity can fluctuate — with illness, medication, time of day or stress — so a finding on one occasion does not settle the question for the next.
Testamentary capacity, the capacity to make a will, is assessed under a separate and older common-law test from Banks v Goodfellow rather than the Act. That is why an assessment carried out for one purpose does not automatically answer the other.