More precisely, Leibniz argues that God created the world soperfectly that each substance acts according to its own law ofunfolding and is at the same time in perfect harmony with all otherssubstances; further, that the mind has a distinct point of view of theworld by virtue of its being the center of some mass (body), and thatthe law of unfolding of the mind is in accord with the laws of thecorporeal machine. He puts this most succinctly in his 1695 essay,A New System of Nature, in which he effectively presents afive-step argument for pre-established harmony:
Leibniz is often put in the camp of rationalists and opposed to theempiricists (for example, Locke, Berkeley, and Hume). While there aregood grounds to be unhappy with this standard textbook distinction,Leibniz does fit the bill in two important respects: he is arationalist insofar as he holds to the Principle of Sufficient Reason,and he is a rationalist insofar as he accepts innate ideas and deniesthat the mind is at birth a tabula rasa or blank slate. Interms of Leibniz's classical allegiances, it is interesting to seethat in the realm of metaphysics, he often couched his philosophy inAristotelian (and Scholastic) terms but that in the realm ofepistemology, he was a fairly open Platonist – at least in termsof the existence of innate ideas. Indeed, in the opening passages ofhis New Essays on Human Understanding, his book-lengthcommentary on Locke's Essay Concerning Human Understanding,Leibniz explicitly aligns himself with Plato on the fundamentalquestion of the origin of ideas. (A VI vi 48/RB 48)
Unlike most of the great philosophers of the period, Leibniz did notwrite a magnum opus; there is no single work that can be saidto contain the core of his thought. While he did produce two books, theTheodicy (1710) and the New Essays Concerning HumanUnderstanding (finished in 1704 but not published until 1765), thestudent of Leibniz's thought must piece together Leibniz'sphilosophy from his myriad writings: essays published in scholarlyjournals and in more popular journals; unpublished works left abandonedby their author; and his many letters. Moreover, many ofLeibniz's writings have not yet been published. The authoritativescholarly version of Leibniz's works, the Akademie edition, hasthus far only published his philosophical writings from 1663 to 1690;in other words, only half of his writing life has beencovered. And the mere act of dating pieces often depends upon carefulanalysis of the paper Leibniz wrote on and watermarks and so on.(Hence, for example, the important short work, Primary Truths,which, because of its content, was often thought to date to 1686 (as inAG), has recently been redated by the Akademie editors to 1689 becauseof a watermark.) Piecing together Leibniz's philosophy into asystematic whole is made more difficult because Leibniz seems to havechanged or at least refined his views on a number of issues over thecourse of his career and because he was always very aware (some mightsay too aware) of the audience for any of his writings.
Leibniz has several straightforwardly metaphysicalreasons for denying that the mind could be a tabula rasa.First, and most obvious, since there can be no genuine causalinteraction among substances, then there could be no way that all ourideas could come from experience; indeed, no ideas could, strictlyspeaking, come from experience. (Leibniz will, however, adopt a moreliberal understanding of sense experience, so that this is not mootedtout court.) But, second, and rarely remarked upon, Leibnizbelieves that the view that our minds are blank slates at birth violatesthe Principle of the Identity of Indiscernibles. In short, PII worksagainst qualitatively identical physical atoms and againstqualitatively identical (because blank) souls. Further, in onetelling passage, he shows us the metaphysical underpinnings of theempiricist view that he finds so objectionable. He writes,“Experience is necessary, I admit, if the soul is to be givensuch and such thoughts, and if it is to take heed of the ideas that arewithin us. But how could experience and the senses provide the ideas?Does the soul have windows? Is it similar to writing-tablets, or likewax? Clearly, those who take this view of the soul are treating it asfundamentally corporeal.” (A VI vi 110/RB 110) Locke famouslyentertained the possibility of “thinking matter”, andLeibniz found such a thesis abhorrent. Throughout his career, Leibnizexpresses no doubt that the human mind or soul is essentiallyimmaterial, and Locke's skepticism about the nature of substanceis fundamentally at odds with Leibniz's most deeply heldphilosophical commitments. But, of course, the consequence of this isthat Leibniz seeks to undermine Locke's position with respect tothe origin and nature of ideas. That the mind, according to Leibniz,must be essentially immaterial has been shown above in thesection on metaphysics. But Leibniz does have a particular argument forthe mind's immateriality or against its mechanism that concernsthe nature of thought and ideas. This is his famous metaphor of a mill,which comes forth both in the New Essays and theMonadology. According to Leibniz, perceptions cannot beexplained in mechanical or materialistic terms. Even if one were tocreate a machine to which one attributes thought and the presence ofperceptions, inspection of the interior of this machine would not showthe experience of thoughts or perceptions, only the motions ofvarious parts.
This changed, however, in 1672, when Leibniz was given the singlemost important opportunity of his life: the Elector of Mainz sent himon a diplomatic mission to Paris, the center of learning and science atthe time. Leibniz was able to stay in Paris for four years (with abrief trip to London in 1673), during which time he met many of the major figures of the intellectual world, among them , , and, most important, the Dutch mathematician and physicist, Christiaan Huygens. It was he, “the great Huygenius” (as John Locke wouldcall him in the Dedicatory Epistle to his Essay Concerning HumanUnderstanding), who took Leibniz under his wing and tutored himin the developments in philosophy, physics, and mathematics. Not onlywas Leibniz able to converse with some of the greatest minds of theseventeenth century while in Paris, he was also given access to theunpublished manuscripts of Descartes and Pascal. And, according toLeibniz, it was while reading the mathematical manuscripts of Pascalthat he began to conceive what would eventually become hisdifferential calculus and his work on infinite series. In this time,Leibniz also designed a calculating machine able to perform addition,subtraction, multiplication, and division (see the Other InternetResources for a picture). And his trip to London in 1673 was meant inpart to present his designs to the Royal Society.
Famous as a playwright and essayist, Voltaire’s Candide is the book where he tries to point out the fallacy of Gottfried William von Leibniz's theory of Optimism.