A Will is not a contract because it does not represent a promise to perform a service or execute an action for another person (including corporations).  It is solely an allocation of a person’s wealth on death.  A contract, such as a Trust, implies the potential for an adversarial relationship if one of the parties does not perform as promised. In a Trust the parties may be the Trustors and the Trustees. Therefore, an adversarial relationship potentially exists between the parties.  Because of this potential adversarial relationship, each of the parties must be able to understand the consequences of their actions vis-à-vis being in default. This requires each party to be able to understand with meaning (mental competency) the terms and conditions of the contract which may themselves be complex and require multiple steps. For this reason the mental state required to enter into a contract requires an understanding of consequences and an ability to understand complex meanings contained within the contracted obligation, neither of which is explicit or implied in the execution of a Will.

Mental competence to enter into a contract has a higher threshold than mental competence to execute a Will.  It is therefore possible for a person to retain testamentary capacity but not be competent to execute a Trust.