In the author's description of peripherals, I did not see a discussion of buttons. Buttons provide stimuli to systems as well - usually in the form of "Button-Down" events followed by "Button-Up" events. Different buttons (such as the left button or right button on mice) can generate different events. Buttons are commonly used in combination with mice and trackballs to provide a "selection" capability. Selection is commonly done by moving the cursor (using the trackball or the mouse) to a specific area of the screen ("pointing") and then indicating the position of interest ("selecting") by a button press (a 'button-down' event).

Is it possible to use a mouse as a selection device without the use of a button? If so, how (and provide an example)? If not, why not?