I will assume the "monads" have positions in space, with coordinates x and y (and perhaps z).

The distance, as commonly defined in Euclidian space, for two such points, is abs(monad1.x - monad2.x).

For two or more dimensions, use Pythagoras. For example, in 5 dimensions, the distance would be sqrt((m1.x - m2.x) ^ 2) + (m1.y - m2.y) ^ 2) + (m1.z - m2.z) ^ 2) + (m1.v - m2.v) ^ 2) + (m1.w - m2.w) ^ 2)). Or whatever you choose to call the coordinates.

I am assuming that you may want to do some pretty abstract stuff, and that you may not want to restrict yourself to the standard 3 dimensions.

In mathematics, many other "distance functions" have been defined; the usual expectations from a "distance function" is that the distance is always zero or positive; also that it satisfies the following three criteria:

1. distance(x, x) = 0

2. distance (x, y) = distance(y, x)

3. distance(x, y) + distance(y, z) >= distance(x, z)

A well-known example of such a "distance function" that satisfies the above requirements, but that is contrary to the intuitive (or Euclidian) "distance" is the answer to the question, in the game of chess, "How many moves does it take the Knight to go from square 'a' to square 'b'"? For example, two adjacent squares (like A1 and A2) have a distance of 3, according to this definition; two diagonally-adjacent cells (like E1 and F2) have a distance of 2. Axioms 1-3 are satisfied.

>Let me tell you.
>
>First let me get straight that "No Windows" in the philosophy of the 18th Century meant something different than it does today amongst loyal Mac users.
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>Let's say I have 2 monads, each with a dimension x.
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>monad1.x = 5
>monad2.x = 20
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>Let's say we wanted to find out the distance between these monads.
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>Obviously, we would suggest monad2.x - monad1.x which is 15.
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>But the monads are window-less. When I say monad2.x, I just looked directly into the monad.
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>That's not allowed.
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>How then do we determine the distance between them?
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>More monads, operating in such a complex way as to compound and act as an observer, complete with sense organs and a brain.
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>The relative space between the windowless monads can be determined from relational data encoded in the brain of an observer.
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>Please, take a look at my paper and free tools, and do pass on my work. If for no other reason than because Leibniz isn't around to champion his vision himself.