Short answer. Yes. Article 1328 of the Civil Code states that contracts entered into during a lucid interval are valid. A mental illness diagnosis does not automatically invalidate every contract that person signs — what matters is whether the person had lucid, rational understanding at the specific moment the contract was made.
What the law says
Contracts entered into during a lucid interval are valid.
Civil Code, Article 1328 — Lucid Intervals; Drunkenness; Hypnosis. Read the full provision →
The lucid interval rule
Article 1328 recognises that mental illness does not operate continuously at full force at every moment. A person with a diagnosed mental condition may have periods of clarity — lucid intervals — during which they understand what they are doing, can exercise independent judgment, and can form genuine consent. The Civil Code treats contracts made during such intervals as valid, full-stop. The diagnosis alone is not enough to nullify the contract; the challenger must show that the specific moment of signing was not a lucid interval.
What a lucid interval actually means
A lucid interval is not simply a period when a person appears calm or cooperative. It is a period during which the mental illness has temporarily receded sufficiently for the person to appreciate the nature and consequences of their actions and to give meaningful, rational consent. Courts look at whether, at the time of signing, the person understood what they were agreeing to, who the other party was, and what they were receiving or giving in return. Evidence from medical professionals about the pattern and frequency of lucid intervals is often central to this inquiry.
The contrast: drunkenness and hypnosis
Article 1328 also addresses two other states that affect consent, but with a different outcome. Contracts made in a state of drunkenness or during a hypnotic spell are not void — they are voidable. This means they are valid unless annulled. The distinction matters because a voidable contract must be actively challenged within a period, whereas a void contract has no legal effect from the start. Mental illness contracts signed during a lucid interval are simply valid; they are not even voidable unless the person lacked lucidity at the critical moment.
Challenging such a contract
If someone wants to challenge a contract signed by a person with a mental illness, the burden is to prove that the person was not in a lucid interval at the time. Evidence might include medical records showing a florid episode on the date of signing, witness testimony about the person's state of mind, communications that reveal confusion or irrationality around that time, or expert psychiatric testimony. 'He has always been mentally ill' is not, by itself, enough — the law gives validity to the contract if reason prevailed at the moment of signing.