The principle has a variety of expressions, all of which are perhaps best summarized by the following:
A sufficient explanation may be understood either in terms of reasons or causes for like many philosophers of the period, Leibniz did not carefully distinguish between the two. The resulting principle is very different, however, depending on which interpretation is given.
In fact Leibniz opposed fatalism and had a more nuanced and characteristic version of the principle, in which the contingent was admitted on the basis of infinitary reasons, to which God had access but humans did not. He explained this while discussing the problem of the future contingents:
We have said that the concept of an individual substance [Leibniz also uses the term haecceity ] includes once for all everything which can ever happen to it and that in considering this concept one will be able to see everything which can truly be said concerning the individual, just as we are able to see in the nature of a circle all the properties which can be derived from it. But does it not seem that in this way the difference between contingent and necessary truths will be destroyed, that there will be no place for human liberty, and that an absolute fatality will rule as well over all our actions as over all the rest of the events of the world? To this I reply that a distinction must be made between that which is certain and that which is necessary. (§13, Discourse on Metaphysics)
Without this qualification, the principle can be seen as a description of a certain notion of closed system, in which there is no 'outside' to provide unexplained events with causes. It is also in tension with the paradox of Buridan's ass.
The principle was one of the four recognised laws of thought, that held a place in European pedagogy of logic and reasoning (and, to some extent, philosophy in general) in the eighteenth and nineteenth century. It was influential in the thinking of Leo Tolstoy, amongst others, in the elevated form that history could not be accepted as random.
If a new state of one or several real objects appears, another state must have preceded it upon which the new state follows regularly.
If a judgment is to express a piece of knowledge, it must have a sufficient ground. By virtue of this quality, it receives the predicate true. Truth is therefore the reference of a judgment to something different therefrom.
The position of every object in space and the succession of every object in time is conditioned by another object's position in space and succession in time.
Every human decision is the result of an object that necessarily determines the human's will by functioning as a motive.