every shape is made of string

"In a black and white drawing the spaces are all white and all are bounded by black lines; in most oil paintings the spaces are multi-coloured and so are the boundaries; you cannot imagine a boundary line without any content, or a content without a boundary lines....therefore,when I speak of significant form, I mean a combination of lines and colours( counting white and black as colours)"
   (Bell "Art")
Seeing a line as a boundary line of content is perceptive seeing.
Seeing a line itself without content is sensitive seeing.
We can see an object and its two dimensional reproduction in the same way on sense level: the retinal images of both are two dimensional, which is why we can establish unified art theory for every thing we see.

 When we say that these shapes are same, it means we move the eye on both shapes in the same way.
Imagine the first one is made of string and the second of  a piece of wood.
We still can say that we see these things in the same way: we move the eye along the outline.
Seeing a shape is tactile: the finger can sense a shape along the contour.

Let me explain the eye movement using the model below: the line becoming wider and finally we see only one edge.

We can see a line because it has thickness, which means the line has two edges.
When the line is narrow both edges are seen at once which is the same as we see one edge.
When the line becomes wider we see the two edges, which is why we see the area.

Everything we see becomes a design network on the retina.
Every network is made of string.
The simplest network is a string and a closed string is a loop.
It can be said that a string is an element.
As atoms make a molecule and may become compound, string may compose more complicated patterns.

We can also start with a network as an element.
In this case a string or a loop is a broken network.
A network has countable strings and dots( meeting points of strings).
When two strings meet at a point, the result is a string.
Only when more than three strings meet at a point a dot appears
A dot is expressed as D(N), N=3,4,5....: dot with N strings.
As every design is made of string and dot, the eye counts the number of the strings and the dots when we see it.The model can be drawn on grid having the characteristics of proportion and angle.
For example D(0) type has only two models: 

Let me show some examples; there are many D(0) with two loops inside in Arp's paintings.

All the shapes belong to the same D(0) type with two loops inside:

Some of them look like creatures with two eyes, which is perception level of seeing.
Piaget wrote that young children acquire topological level of seeing at very early stage.
.I consider this is the beginning of sensitive seeing.
Associating two eyes which  consists of sensing and referring comes later.

The model works to classify alphabet:

There are ten types of models in this group.

D(0)

D(3)

2 D(3)

 D(4)

Not only the alphabet but any design has a string model.
For example using the two models above this new model can be made.

Imagine this is stretchable string then it is easy to make Arp's work bellow.

Many modern paintings originated from Cubism (including Constructivist painting, Dada ism painting) have distinctive types of string model, which suggests they belong to the same group.

"I wonder.... whether the appreciators of art and of mathematical solutions are not even more closely allied.... I have been inquiring why certain combinations of forms move us; I should not have travelled by other road had I enquired, instead, why certain combinations are perceived to be right and necessary, and why our perception of their rightness and necessity is moving."
     ( Bell)
Bell is showing which  direction I should go.