Describing Objects in the First Dimension The "third" dimension is the one we experience day-to-day. An example of the "second" dimension is the shadow of a three-dimensional object. How do we describe the "first" dimension? What would be an example of matter in the first dimension?
 A: You say:

The "third" dimension is the one we experience day-to-day

but this is not so. We experience three (spatial) dimensions, but there is no distinction between the first, second and third dimensions. For example I might choose the first, $x$, and second, $y$, dimensions to be horizontal and the third $z$, dimension to be vertical. I live in the UK, but suppose a friend in the US does the same, our dimensions would be different:

My first, $x$, dimension isn't the same as my friend's $x$ dimension. So which of us is correct? Well, neither of us. There isn't a unique first dimension; we can arrange our dimensions at whatever angles we want - it doesn't make sense to talk about a first, second and third dimension because the distinction is a matter of choice. All we can be sure of is that there will be three dimensions.

An example of the "second" dimension is the shadow of a three-dimensional object.

We can image a plane cutting through our three dimensional space, and this plane would be a two dimensional object. However our 2D plane is a purely mathematical construct and no 2D objects really exist.

How do we describe the "first" dimension?

If instead of a plane I draw a line, then the line is a 1D object. However, like the 2D plane, this is just a mathematical construct.

What would be an example of matter in the first dimension?

The point I've attempted to make above is that the universe and everything in it is three dimensional, so it doesn't make physical sense to talk about matter constrained to lie in one dimension.
