In order to exist in society, we have to assume that the vast majority of people are sane.
In order to eat a slice of pizza, you have to assume that no one in the vast supply chain that provided each of the ingredients and proto-ingredients had snapped and decided to dump arsenic or anthrax or polonium into their vat.
You also have to assume that no one sprinkled poison onto your slice while your back was turned, or when you blinked between bites.
You have to assume that there isn't someone lurking behind you with a knife, or that there isn't a bomb under your chair, or that a sniper isn't targeting you from a half mile away.
While we're throwing caution to the wind, let's also assume that sending signals down the nerves in your arm will cause it to move the pizza to your mouth, and not the fork to your eyeball. Let's assume that the information being delivered via our senses corresponds to reality, and that we can make decisions based on it. Let's assume that our memories of the past five minutes also correspond to reality, and that the next five minutes will proceed logically from events in the present. Let's assume that the universe will continue to exist, rather than spontaneously ceasing.
That's a lot of assumptions.