 A modally strong version of the PSR will take the Principle as necessary and obtaining in all possible worlds, while a weak modal version will present the Principle as merely contingently true. Another distinction can be drawn between a factive, as opposed to merely regulative, version of the Principle. A regulative version of the PSR would consider it as a condition for intelligibility (on a par with the Law of Non-Contradiction) and thus as guiding our studying of nature. The factive version simply states that the Principle is true in actuality (or even in all possible worlds). The regulative and the factive versions differ in terms of allowing for the falsification of the principle. The factive version could be easily refuted by a single counter-example. A proponent of the regulative variant of the PSR would argue that an empirical falsification of the PSR makes as little sense as an empirical falsification of the Law of Non-Contradiction.  Encountering a fact which seems to have no explanation, the proponent of the regulative variant would respond by insisting that we must keep searching for an explanation.