In probability theory, a probability must always be between 0 and 1, inclusive. This means:
- It can be 0 (indicating an impossible event).
- It can be 1 (indicating a certain event).
- It can be any number in between (e.g., 0.5).
Now, let's evaluate the given numbers:
- 0.5: This is a valid probability.
- -0.000001: This is not a valid probability since it is negative.
- 0: This is a valid probability (indicating an impossible event).
- 1: This is a valid probability (indicating a certain event).
Therefore, the number that cannot be a probability is -0.000001.