When designing a graphical screen layout, there is great reliance upon conventional interpretations of the symbols and placement. Much of the discussion about the use of affordances is really addressing conventions, or what I call cultural constraints. These are powerful design tools, so let's be clear where each is being used. Locking the mouse button when clicking is not desired would be a physical constraint. Restricting the cursor to exist only in screen locations where its position is meaningful is a physical constraint.
Logical constraints use reasoning to determine the alternatives.
Thus, if we ask the user to click on five locations and only four are immediately visible, the person knows, logically, that there is one location off the screen. Logical constraints are valuable in guiding behavior. It is how the user knows to scroll down and see the rest of the page. It is how users know when they have finished a task. Cultural constraints are conventions shared by a cultural group.