Philosophy (general) Given that experience is argued to be the foundation of knowledge (according to Locke) how - if at all - does Locke make room for what Leibniz would call 'necessary truths'?

Gottfried Leibniz made many criticisms of the work of John Locke, while acknowledging its sophistication and importance, observing that 'although the author of the Essays says hundreds of fine things which I applaud, our systems are very different' (Leibniz, 1982, p. 47). There is indeed a philosophical gulf between the two thinkers. Locke does not believe human beings can have any access to accurate knowledge of the actually existing reality of things, their 'real essence.' Only through the words we use to stand for things do we have any relationship to those things:

Nor indeed can we rank and sort things, and consequently (which is the end of sorting) denominate them, by their real essences; because we know them not. Our faculties carry us no further towards the knowledge and distinction of substances, than a collection of those sensible ideas which we observe in them. (bk. III, chap. VI, para. 9)

This is a fundamentally different position from that of Leibniz, who holds that the way a word is used to refer to an object is part of our understanding of the object's real essence, and that its real essence can be understood through its observable properties. For Leibniz the 'real essence' of a thing is itself a reflection of the 'necessary truths' knowledge of which is innate to the human mind; for Locke there can be no human apprehension of 'real essences' at all, which there surely would be if 'necessary truths' have any actual existence. Our conclusion must be, therefore, that Locke offers no room for what Leibniz calls 'necessary truths.'

Locke is an empiricist, arguing that human knowledge is constructed from experience through the apparatus of the senses, while Leibniz is a rationalist, emphasizing the role of reason rather than sensory experience in the construction of a picture of the world. John Locke asserts clearly in An Essay Concerning Human Understanding that none of the knowledge we possess is innate, that the mind begins as a blank sheet of paper - 'white paper, void of all characters, without any ideas' (Locke, 1690, bk. II, chap. I, para. 2) - which sensory experience fills in with information, and it is only upon this information that the mind can act and so develop all the capacities we consider as intellectual:

The senses at first let in particular ideas, and furnish the yet empty cabinet, and the mind by degrees growing familiar with some of them, they are lodged in the memory, and names got to them. Afterwards, the mind proceeding further, abstracts them, and by degrees learns the use of general names. In this manner the mind comes to be furnished with ideas and language, the materials about which to exercise its discursive faculty. (Locke, 1690, bk. I, chap. I, para. 15)

This is a fundamentally different position from that of Leibniz, who holds that this process does indeed function in human beings as it does in animals - 'men act like the lower animals, resembling the empirical physicians, whose methods are those of mere practice without theory' - but that this is only part of the explanation for human understanding. The element this empirical explanation disregards is precisely that rational capacity which separates people from animals and makes human nature what it is: 'it is the knowledge of necessary and eternal truths that distinguishes us from the mere animals and gives us Reason and the sciences, raising us to the knowledge of ourselves and of God. And it is this in us that is called the rational soul or mind' (Leibniz, 1698, para. 29). Such a view of human nature would be unsustainable without a conception of innate ideas which themselves partake of what...

Furthermore, without this innate knowledge of necessary truths that gives rise to the human capacity for reason, the process of experiencing, learning and understanding outlined by Locke would have nothing on which to act, for:
reflection is nothing but attention to what is within us, and the senses do not give us what we carry with us already. In view of this, can it be denied that there is a great deal that is innate in our minds, since we are innate to ourselves, so to speak, and since we include Being, Unity, Substance, Duration, Changes, Action, Perception, Pleasure, and hosts of other objects of our intellectual ideas? And since these objects are immediately related to our understanding and always present to it... is it any wonder that we say that these ideas, along with what depends on them, are innate in us? (Leibniz, 1982, p. 51)

Leibniz thus poses a question to Locke, and to all empiricists: if it is the action of sense experience on the mind that gives rise to human thought and reflection, upon what is it acting if not upon some innate qualities of mind? For Leibniz the concept of 'necessary truths' is essential, as is the related concept that such truths are innately present.

Locke, by contrast, seems to argue that any such concept is, literally, meaningless. A necessary truth, he claims, would by its nature receive universal assent, but if there are human beings who are unable to give that assent (the examples he gives are 'children and idiots') then they clearly cannot be aware of those truths, so how can they be innately present?

A it seeming to me near a contradiction to say, that there are truths imprinted on the soul, which it perceives or understands not: imprinting, if it signify anything, being nothing else but the making certain truths to be perceived. For to imprint anything on the mind without the mind's perceiving it, seems to me hardly intelligible. If therefore children and idiots have souls, have minds, with those impressions upon them, they must unavoidably perceive them, and necessarily know and assent to these truths; which since they do not, it is evident that there are no such impressions. For if they are not notions naturally imprinted, how can they be innate? And if they are notions imprinted, how can they be unknown? (Locke, 1690, bk. I, chap. I, para. 5)

Such a position would seem categorically to rule out any real existence for Leibniz's 'necessary truths'. This does not mean, however, that there is no room in Locke's scheme of things for transcendent notions. Locke's rejection of innate ideas would appear to have dangerous consequences for religion and morality, but he was not an atheist and was clear about the existence of God and the qualities which God had granted human beings for apprehending and understanding the truth, and that it was the will of God that those qualities should be so used. The variety of opinion possible on such issues as right moral conduct was, for Locke, evidence of this intention on the part of God; eternal and universally agreed ideas on morality would undermine God's purposes in giving human beings the ability to work out those ideas for themselves:

grant the existence of God is so many ways manifest, and the obedience we owe him so congruous to the light of reason, that a great part of mankind give testimony to the law of nature: but yet I think it must be allowed that several moral rules may receive from mankind a very general approbation, without either knowing or admitting the true ground of morality; which can only be the will and law of a God, who sees men in the dark, has in his hand rewards and punishments and power enough to call to account the proudest offender. (Locke, 1690, bk. I, chap. II, para. 6)

Locke goes on…