>Looking through the last essay of Montaigne’s Essays, III 13 ‘Of Experience’, which I’m using as the basis of next week’s Introduction to Philosophy lecture, I was struck by the discussion of the dissimilarity if events and things. It looks very close to Leibniz’s ‘principle of indiscernibles’ (briefly discussed in Discourse on Metaphysics 9, a slightly more lengthy discussion in ‘On the Principle of Indiscernibles’), though Montaigne himself does not use the word ‘indiscernible’.
Montaigne’s comments refer to the impossibility of there being two identical things, even eggs and the back of playing cards can be distinguished by someone with enough knowledge and experience. What Montaigne draws from this is partly about law, and the problem of a judge applying laws and precedents when every case must be different. The ideal would be a natural state, or what Montaigne presumes to be natural, in which the judge makes judgements without regard to law and precedent. Montaigne also refers to the confusion of the human mind in the innumerable different perceptions and interpretations it can have, which is another form of indiscernibility: that no two perceptions or interpretations can be identical.
Leibniz argues that no two substances can be distinguished purely by location in time and space. The argument ultimately refers to the basic principles that all the predicates attached to a substance inhere in it as the substance is the subject of the predicates. One consequence of that for Leibniz is that the relations of a substance with anything else cannot be outside of that substance. A relation in space and time must refer to some inner part of the substance. There is no such thing as existence of a thing separate from its essence. What exists must have more in its essence than what does not exist, and must be more perfect. He also directs the principle against atomism (corpuscular philosophy) since atoms are presumed to be identical. He sees the world as being ordered by the principle of indiscernibles in a way that reduces confusion since it apparently shows that mathematical relations and atoms cannot be the basis of the universe. It is the relations between substances, and between substances and God, in which everything is mirrored in everything else. This is deep order in the universe rather than unbearable complexity and differentiation for Leibniz.
The contexts in which Montaigne and Leibniz discuss the impossibility of identical things differ, but there is also continuity. I did not make deep investigations into this, but I did check the Stanford Encyclopaedia of Philosophy (a very reliable source) and did a bit of Googling. Nothing came up, so I can presume that the connection has not been widely discussed, since I did not find any discussion at all. I would guess both Leibniz and Montaigne were referring to earlier texts, but I don’t know what those would be, and again I presume there is not a great deal of work on this, though surely someone somewhere has looked into this.
I would say there’s a case for taking Montaigne as the ‘founder of modern philosophy’, rather than Descartes, and that this issue of indiscernibles confirms that.