This is one of the basic requirements for the existence of a contract. It is the main motive that makes the parts obligate one against another. The referred motives have no legal relevance as well as they licit; the cause is the stereotyped motive in contracts. But when the motive is not licit, it becomes relevant and can cause the nullity of the contract. (Travezaño)

The Nature of Consideration and Rules that govern Consideration

In order for the cause to be considered it needs to exist, be licit and true (the parts need to be after the cause celebrated).

Illegality

A contract can be considered illegal when it is not considered to have a cause. Also, it will be considered illicit when it is seen to be against imperative or moral laws. (Wolters Kluwer)

Contract Terms

Contract terms are stated through the different articles from the Civil Code.

• When the terms are clear, contractors will follow the literal meaning; but when they seem contrary to the intention of the contractors, intention will prevail over words.
• In order to judge the intention of contractors, it should be attended the acts of the parts.
• It should not appear in the contract different things to those about which the interested parties contacted.
• If any part of the contract admits more than one meaning, it should be understood the one which is more adequate for the effect to be produced.
• The contract clauses should be interpreted one for another, so that in case there is any doubt the rest of the clauses give sense to it.
• Words with different meaning will be understood according to the meaning which is more fitting to the contract.
• Interpretation of any doubtful part of the contract should not favour the contractor responsible for creating that part.
• When it becomes impossible to solve the doubts by the stablished means, if they fall over accidental circumstances and are free, they will be solved in favour of the lower transmission of rights and interests. In case it is not free, it should be solved looking for the highest reciprocity of interests. (Boletín Oficial del Estado)