To presume means to believe something is true without having proof, usually because it seems likely or reasonable. When you presume your friend is home because their bike is in the driveway, you're making an educated guess based on evidence, but you don't know for certain until you knock on the door.

Presuming is different from wild guessing. If dark clouds fill the sky, you might presume it will rain soon. That's a reasonable assumption based on what you observe. But presuming can lead you astray: maybe those clouds will blow past without dropping a single raindrop.

In everyday conversation, people often say “I presume” when they want to show they're not completely certain about something. A teacher might say, “I presume you've all finished your homework,” meaning she expects it's done but hasn't checked every assignment yet.

The word carries a slight sense of taking something for granted. If someone acts overly familiar with a person they just met, you might say they're presuming too much about the relationship. When detectives investigate a crime, they try hard not to presume anything about what happened, because presuming the wrong thing could lead them away from the truth. Good thinking means knowing the difference between what you know and what you merely presume.