Certainty is the state of being completely sure about something, without any doubt. When you have certainty, you know something is true or will happen, as clearly as you know that dropping a ball will make it fall to the ground.

Certainty feels different from just thinking something is probably true. If you're certain your birthday is next Tuesday, you've checked the calendar and you know. If you think it's probably next Tuesday but aren't quite sure, that's not certainty. That's possibility or likelihood.

Some things feel like absolute certainty: mathematical facts like 2 + 2 = 4, or the strong expectation that the sun will rise tomorrow. Other certainties come from evidence and experience. A scientist may feel very close to certainty about a hypothesis after years of careful experiments that all point to the same conclusion.

But certainty can also mislead us. Someone might feel certain they're right about something when they're actually wrong. They confuse strong confidence with true certainty. Careful thinkers try to notice the difference between what they can be certain about and what they merely believe or hope is true.

When something is a certainty, it’s guaranteed to happen: “After studying every night for a month, her good grade was a certainty.”