Thank you for your question. I look forward to working with you to provide you the information you are seeking for educational purposes only.
For starters, PR Code Section 5141 provides that "[a] person who by an act or omission causes damage to another through fault or negligence
shall be obliged to repair the damage so done. Concurrent imprudence of the party aggrieved does not exempt from liability, but entails a reduction of the indemnity." See: P.R.Laws Ann. tit. 31, § 5141. The PR law does not specify knowledge, the PR Supreme Court has ruled several times on the issue and it has evolved over time.
The OLD PR rule was that the defense of a lack of constructive knowledge was not viable in § 5141 cases. See: Aponte v. Melendez, 87 P.R.R. 619 (1963). The court in Aponte stated that shopowners must keep public areas free of "slippery spots," while ignoring the language in previous cases concerning actual or constructive knowledge.
The Aponte case imposed a strict liability or, as the Puerto Rico courts sometimes say, a res ipsa loquitur standard upon store owners, in which a dangerous condition in the store led to a per se finding of negligence. See: Dopico-Fernandez v. Grand Union Supermarket, 841 F.2d 11, 15 (1st Cir.1988). Also see: Feliciano v. Escuela de Enfermeras, 94 P.R.R. 509 (1967), where the court imposed strict liability on defendant property owner after plaintiff slipped on wet stairs.
HOWEVER that is not the end of the issue. The US District Court of Appeals, 1st Circuit, had an issue to weigh in on the matter and basically overturned the Aponte line of cases by holding that the PR Court in Cotto v. Consolidated Mutual Insurance Co., 116 D.P.R. 644 (1985), changed the standard. The US 1st Circuit court of appeals has now held the current state of the law in Puerto Rico is that PR Code Section 5141 requires, as an element, an affirmative showing by the plaintiff that the defendant was negligent. This showing, in turn, requires a demonstration that the defendant has either actual or constructive knowledge of a dangerous condition. See: Mas v. United States, 984 F. 2d 527 (1st Cir. 1993).