The sum of your probabilities must add to 1. If you reduce the probability assigned to one theory, the freed probability mass must flow into other theories to preserve the sum.
But why are we assigning probability across a spectrum of competing theories? I thought we were supposed to be assigning probability to the theories themselves.
In other words, P(X) is my best guess at X being true. P(Y) is my best guess at Y being true. In the case of two complex theories trying to explain a particular phenomenon, why does P(X) + P(Y) + P(other theories) need to equal 1?
Or am I thinking of theories that are too complex? Are you thinking of X and Y as irreducible and mutually exclusive objects?
Followup to: The Most Important Thing You Learned
What's the most frequently useful thing you've learned on OB - not the most memorable or most valuable, but the thing you use most often? What influences your behavior, factors in more than one decision? Please give a concrete example if you can. This isn't limited to archetypally "mundane" activities: if your daily life involves difficult research or arguing with philosophers, go ahead and describe that too.