I assume you're going to lunch.
=> I notice it's 12:00 p.m., you've got what appears to be your lunch bag in hand, and so I deduce that you're going to lunch. My assumption is based on deduction.
 I presume you're going to lunch.
=> It's 12:00 and you've got what appears to be your lunch bag in hand, and I notice nothing, not the time, not what you are carrying. I am on my way to lunch, so I presume that you, too, must be going to lunch. My presumption is based on my own experience, on what I think, and not on the facts at hand.
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