Mental capacity has been defined as the ability to make a rational decision, which includes the ability to perceive and appreciate all relevant facts.  Capacity of not necessarily synonymous with sanity.  In legal contexts, no one can be guilty of crime who lacks the legal capacity to commit it.  Moreover, to render a contract binding the parties involved must have had the capacity contract.  In order to dispose of property in a will the testator must have testamentay capacity.  What constitues mental capacity in any given legal situtaiton is governed by the law of parties’ domicle.