Probabilities range from 0 to 1. If a proposition has a probability of 0, then it’s certainly false; if 1, then it’s certainly true. A proposition with a probability of ½ (or 0.5, or 50%) is equally likely to be true as false, and a proposition with a probability of ¾ (or 0.75, or 75%) is three times as likely to be true as false.