"Perception is reality." This quote reminds us that there is always a gap between our perception and others'. And, in design, that matters a great deal. It's helpful to remember. Yet, it's also incomplete. We know that our perception is shaped through the lens of who we are, our past experiences, etc. Hence, my perception is my reality; your perception is your reality; their perception is their reality, etc. 

Think about "sticky notes," what comes to mind?