G.W. Leibniz - The Priniple of Sufficient Reason and his Argument for the Existence of God
from Leibniz, The Monadology (1714):
"... we can find no true or existent fact, no true assertion, without there being a sufficient reason why it is thus and not otherwise, although most of the time these reasons cannot be known to us. ...
"There is an infinity of figures...of minute inclinations....Now, all of this detail implies previous or more particular contingents, each of which again stands in need of similar analysis to be accounted for, so that nothing is gained by such analysis. The sufficient or ultimate reason must therefore exist outside the succession of series of contingent particulars, infinite though this series be. Consequently, the ultimate reason of all things must subsist in a necessary substance, in which all particular changes may exist only virtually as in its source: this substance is what we call God."
from Leibniz, "The Principles of Nature and Grace, Based on Reason" (1714):
"...now we...make use of the great...principle that nothing takes place without a sufficient reason; in other words, that nothing occurs for which it would be impossible for someone who has enough knowledge of things to give a reason adequate to determine why the thing is as it is and not otherwise. This principle having been stated, the first question which we have a right to ask will be, 'Why is there something rather than nothing?'.... Further, assuming that things must exist, it must be possible to give a reason why they should exist as they do and not otherwise.
"Now this sufficient reason for the existence of the universe cannot be found in the series of contingent things....Although the present motion...arises from preceding motion, and that in turn from motion which preceded it, we do not get further however far we may go, for the same question always remains. The sufficient reason, therefore, which needs not further reason, must be outside of this series of contingent things and is found in a substance which...is a necessary being bearing the reason for its existence within itself; otherwise we should not yet have a sufficient reason with which to stop. This final reason for things is called God."
from Leibniz, "On the Ultimate Origination of the Universe" (1697):
"Besides the World, that is, besides the aggregate of finite things, there is some dominant unit...[that] not only rules the world, [but] also makes or creates it. It is superior to the world and, so to speak, beyond the world, and is therefore the ultimate reason for things. Neither in any single thing, nor in the total aggregate and series of things, can the sufficient reason for their existence be discovered. Let us suppose a book...to have existed eternally, one edition having always been copied from the preceding: it is evident then that, although you can account for the present copy by reference to a past copy which it reproduces, yet, however far back you go ...you can never arrive at a complete [explanation], since you always will have to ask why at all times these books have existed, that is, why there have been any books at all and why this book in particular. What is true concerning these books is equally true concerning the diverse states of the world, for here too the following state is in some way a copy of the preceding one (although changed according to certain laws). However far you turn back...you will never discover in any or all of these states the full reason why there is a world rather than nothing, nor why it is such as it is.
"You may well suppose the world to be eternal; yet what you thus posit is nothing but the succession of its states, and you will not find the sufficient reason in any one of them, nor will you get any nearer to accounting rationally for the world by taking any number of them together: the reason must therefore be sought elsewhere. Things eternal may have no cause of existence, yet a reason for their existence must be conceived. Such a reason is, for immutable things, their very necessity or essence; while in the series of changing things, even though this series itself may be supposed a priori to be eternal, this reason would consist in the very prevailing of inclinations. For in this case reasons do not necessitate (that is, operate with absolute or metaphysical necessity, so that the contrary would imply contradiction), but only incline. Hence it is evident that even by supposing the world to be eternal, the recourse to an ultimate cause of the universe beyond this world, that is, to God, cannot be avoided.
"The reasons [sufficient, full, complete] for the world are therefore concealed in some entity outside the world....Thus we must pass from the physical or hypothetical necessity, which determines the later states of the world by the earlier, to something endowed with absolute or metaphysical necessity, for which no reason can be given. For the actually existing world is necessary only physically or hypothetically, but not absolutely or metaphysically....Since therefore the ultimate root of the world must be something which exists of metaphysical necessity, and since furthermore the reason for any existent can be only another existent, it follows that a unique entity must exist of metaphysical necessity, that is, there is a being whose essence implies existence. Hence there exists a being which is different from the plurality of beings, that is, from the world; for it has been granted and proved that the world does not exist of metaphysical necessity."