Take right half of the above image and think about if you see:

a) A vase; or

b) Two faces.

My initial observation was a vase. To see the vase, my mind must assume the black area is the negative space and the white area is the positive space.

If you see the two faces, then you are assuming the white area is the negative space and the black area is the positive space.

On the left side, the vase is much more apparent due to the added detailing. You can still see the two faces, but you may need to change your focus.

The point is, positive and negative space is to some extent subjective. However, this illusion is an extreme demonstration of the differences in perception. Most of the time, the positive and negative space will be obvious and everyone will see the same thing.

For example, in the cloudscape painting earlier in this post, I think it is safe to assume almost everyone will see the clouds as the positive space.