If you want to make completely rational decisions, then you should compute those probabilities to know they actually are below threshold before discarding them.

If you want to make completely rational decisions, then you should compute those probabilities to know they actually are below threshold before discarding them. Considering that the number of possible (albeit improbable) events to consider is practically infinite, this would still take a practically infinite time. What instead we could do is discard lots of possible probabilities without even considering them, because they just look intuitively unlikely enough. In other word, using the so called "cognitive shortcuts".