Anindya wrote:

> but tbh I don't really have much of a feel for what a relation is, beyond the formal definition.

I tend to visualize relations as matrices of 0's and 1's, or matrices of \\(\texttt{true}\\)'s and \\(\texttt{false}\\)'s. Composing relations is just matrix multiplication using \\(\texttt{and}\\) and \\(\texttt{or}\\) instead of \\(\times\\) and \\(+\\).

Profunctors are also just glorified matrices, with composition being done by a version of matrix multiplication.

Another way I imagine a relation is this. A set is a bunch of dots. A relation from a set \\(X\\) to a set \\(Y\\) is as a bunch of line segments: one joining each dot in \\(X\\) to each dot in \\(Y\\) to which its related:

Unlike the usual picture of a _function_:

you probably don't want to imagine _arrowheads_ on the line segments for a relation! You want to be able to just turn around your mental picture of a relation \\(R \colon X \nrightarrow Y\\) and see the transpose \\(R^\top \colon Y \nrightarrow X\\).

Of course each line segment represents a \\(\texttt{true}\\) in the matrix I mentioned earlier, and each _absence_ of a line segment represents a \\(\texttt{false}\\). Composing relations has a very simple pictorial interpretation in terms of these line segments... and you can see that it's the same as matrix multiplication using \\(\texttt{and}\\) and \\(\texttt{or}\\).

I don't know if this helps, but I feel completely happy with my understanding of relations.... probably because I have a bunch of interlocking ways to think about them... so maybe I can make you happy too.

> but tbh I don't really have much of a feel for what a relation is, beyond the formal definition.

I tend to visualize relations as matrices of 0's and 1's, or matrices of \\(\texttt{true}\\)'s and \\(\texttt{false}\\)'s. Composing relations is just matrix multiplication using \\(\texttt{and}\\) and \\(\texttt{or}\\) instead of \\(\times\\) and \\(+\\).

Profunctors are also just glorified matrices, with composition being done by a version of matrix multiplication.

Another way I imagine a relation is this. A set is a bunch of dots. A relation from a set \\(X\\) to a set \\(Y\\) is as a bunch of line segments: one joining each dot in \\(X\\) to each dot in \\(Y\\) to which its related:

Unlike the usual picture of a _function_:

you probably don't want to imagine _arrowheads_ on the line segments for a relation! You want to be able to just turn around your mental picture of a relation \\(R \colon X \nrightarrow Y\\) and see the transpose \\(R^\top \colon Y \nrightarrow X\\).

Of course each line segment represents a \\(\texttt{true}\\) in the matrix I mentioned earlier, and each _absence_ of a line segment represents a \\(\texttt{false}\\). Composing relations has a very simple pictorial interpretation in terms of these line segments... and you can see that it's the same as matrix multiplication using \\(\texttt{and}\\) and \\(\texttt{or}\\).

I don't know if this helps, but I feel completely happy with my understanding of relations.... probably because I have a bunch of interlocking ways to think about them... so maybe I can make you happy too.