From "The Gift of Fear" by Gavin De Becker on p124:
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    Imagine a man arriving for work one morning.  He does not go in the
    unlocked front door where most people enter the building but instead goes
    around to a back entrance. When he sees someone ahead of him use a key to
    get in, he runs up and catches the door before it relocks.  Once he is
    inside the building, he barely responds as a co-worker calls out, "The
    boss wants to see you," "Yeah, I want to see him too," the man says
    quietly. He is carrying a gym bag, but it appears too heavy to contain
    just clothes. Before going to his boss's office, he stops in the locker
    room, reaches into the bag, and pulls out a pistol. He takes a second hand
    gun from the bag and conceals both of them beneath his coat. Now he looks
    for his boss.

    If we stopped right here, and you had to predict this man's likely
    behavior on the basis of what you know, context would tell the tale,
    because to know just one thing changes every other thing: This man is a
    police detective. If he were a postal worker, your prediction would be
    different.