There is a law of nature that seems to be universal, applying to all sorts of situations. It was initially formulated with respect to thermodynamics, the study of heat flow. In that context the second law states that heat flows from hot objects to cold objects with a consequent decrease in the hot object's temperature and increase in the cold object's temperature. The temperature changes reduce the distinction between the hot and cold objects, increasing the entropy of the system.

It is the tendency for entropy to increase over time that is the universal aspect of the second law. This tendency is so ubiquitous that it has been taken to define the direction of time's flow from past to future. The second law indicates which of the possible future states of a system is most likely to survive the passage through now from the future to the past.