Is mousePosition not equal object.transform.position?

I started a new clear project. One plane and one cube on it.

I try now to move the cube to any position where I click with my mouse.

 function Update () {
 
     if (Input.GetMouseButtonDown(0))
     {
         var cube1 = GameObject.Find("Cube1");
         cube1.transform.position.x = Input.mousePosition.x;
         cube1.transform.position.z = Input.mousePosition.z;
     }
 }

But I think I miss the link.

How can I establish the relation between the coordinates of the cube and the coordinate of my mouse? Instead to move the cube 1cm e.g. to the right when I clicked next to cube, the cube moved to whatever my screen size is. So in this case instead from x=0 to x=1 the cube moved to x=585.

Which function reads the correct distance or coordinates related to mouse position?