Capacity to contract means the legal competence of a person to enter into a valid contract. Usually the capacity to contract refers to the capacity to enter into a legal agreement and the competence to perform some act. The basic element to enter into a valid contract is that s/he much have a sound mind.
Certain class of people are exempted from the category of people who are capable of entering into contract:
3. people under the influence of drug;
5. enemy alien.