Heuristical immutability is the property (or a set of properties), that a "mutable code" may or may not have, which essentially is the basic principals behind code's behavior, that do not change as mutable code gains more and more experience.
Human conscious with it memory and the ability to change its "mind", habits and preferences is a good example of mutable code incarnation. However, with all the "great possibility of change" or simply "flexibility" available with human brain, heuristical immutability can co-exist along with flexibility as the property of the mind.
Examples of heuristical immutability in humans are strong beliefs that are not possible to change during the course of man's life. Love of parents, patriotism of a homeland or love for one woman even if the subject of love is no longer around.