  For Wolff,[5] the PC is fundamental, and it is an innate first principle. Methodologically, Wolff starts with the psychological impossibility of believing or judging that something both is and is not. Wolff thus accepts version number 2 above of the PC, and he starts with the first person awareness of one's psychological inability to not accept the PC so presented. This psychological awareness further reveals the logical impossibility of any proposition that asserts that something is both true and false at the same time, and the logical impossibility of a proposition that asserts contraries further reveals the ontological fact that something cannot have contrary properties.  The PC first and primary job, then, for Wolff is to divide impossible concepts and things—concepts and things that embody a contradiction—from possible concepts and things—concepts and things that do not embody a contradiction. Possible concepts for Wolff include both necessary truths and contingent truths.  Thus, the fundamental and first job of the PC for Wolff includes the work of dividing both necessary and contingent truths from impossible truths. As result, in the realm of modality, the PC serves to identify not only necessary truths, but also contingent truths, pace Leibniz for whom, in the realm of modality, the PC serves to identify necessary truths. Once impossible concepts and things are isolated from the rest, the PSR enters to further explain aspects of possible things.  It does so in two ways. First, the PSR acts as a “principle of being” in the sense that there is a sufficient reason to explain the essence of any possible being; there is a sufficient reason why a being's essence is as it is. This role for the PSR is relevant in the case of both necessary beings and contingent beings, and on this point, Wolff once again divergences from Leibniz in that the latter often (though not always) restricts the role of the PSR to that of providing an explanation for why contingent beings, though not necessary beings, are as they are. For Leibniz (usually), the PC explains why necessary beings (and truths) are as they are. Still, the divide between Wolff and Leibniz on this point may be narrower than it seems, because for Wolff, the sufficient reason why a necessary being is as it is just is that the PC establishes this. The second way in which the PSR enters the picture to further explain aspects of possible things for Wolff is that it acts as a “principle of becoming”. In this role, the PSR gives the causes or grounds for why a possible being becomes actual as opposed to remaining possible but non-actual.