Gravitation attraction of inner portion problem

[mooncrater]

Homework Statement

While solving a question on gravitation I had to consider a very thin spherical layer of a uniformly dense sphere. It's written in the solution that :
"Then the considered layer is attracted to the part of the sphere lying within it (outer part doesn't act on the layer)

Homework Equations

The Attempt at a Solution

I have problem in the bold part. Why won't the upper part not attract the layer? Why only the inner layer will do so?

 

[Doc Al]

Look up Newton's Shell theorems.