Descartes: Good evening, Leibniz.
Leibniz: Good evening, Descartes.
Descartes: Leibniz, I’ve been trying to understand the true nature of reality… How can I know that this chair I believe to be sitting on is truly there, or is truly brown and solid? How can I know that I am truly here?
Leibniz: Interesting. How do you think we can know?
Descartes: First I began questioning the extent to which matter can interact with mind.
Leibniz: Well, first I believe we must question: what is matter?
Descartes: Yes, we must. I believe whatever has spatial extension is matter and matter, by definition, is that which has spatial extension. Spatial extension, then, constitutes matter.
Leibniz: And what is spatial extension?
Descartes: It is length,
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Why not some other quality of matter? It does not tell us how to distinguish an apple from an orange. Apples and oranges have similar length, breadth and thickness but I tell them apart by their taste, texture and colour. There is more to things than their geometrical aspect.
Descartes: Colour, smell and taste are sensations that cannot exist without our perceptions of the objects, or matter, themselves. They are sensations existing in my mind: sense data, if you will. They are too volatile to constitute matter but are an extension of it conceived in our minds.
Leibniz: But I cannot imagine an apple without thinking of its colour and
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Descartes: But if we were to light the candle the wax would be very different. Leibniz: Yes, it would. It would be soft – melted even -, transparent and liquid more fragrant.
Descartes: You see? All the properties have changed but it remains the same wax. Therefore, extension is the fundamental property of physical things. We cannot rely on our sense data, such as colour and texture. With all this in mind, the mind and body must be separate from one another. The mind cannot be extended so there must be a body – and matter - that is extended. Leibniz: Descartes, you must be mistaken. If the mind and body were completely distinct, then how can the mind and body interact with one another? Descartes: True… If I wanted to lift my arm up, I would have to have some sort of connection or interaction between my mind and my body… but surely any causal interaction between them is impossible?! Leibniz: Let me see if I can help. This problem suggests that there must be some pre-established harmony between body and mind and all substances. If we look towards physics and mathematics we can see that the universe is entirely made up of one element given the general name of a monad, a simple substance which is one, has no parts and is therefore indivisible. These monads are the first elements of every composed