[dragonwriter]

> There is no number seven. I mean there seven trees, seven stones, yet there is no number seven.

There aren't trees or stones any more than there is the number seven: objects (other than elementary particles, maybe) are as much mental constructs as numbers.

[placebo]

Why stop at elementary particles? All objects are mental constructs of consistencies our brains detect in data that our senses pick up.

Humans have a great advantage in picking up on what I'd call meta-consistencies (which is basically what abstraction is) - creating a new mental construct by detecting consistencies not only in what the senses pick up, but detecting consistencies in those consistencies, or "thoughts about thoughts" if you will. This gives you numbers and other abstract concepts that enable far more powerful manipulation.