Time is the transfer (flow) of energy by matter. If you do a mental experiment and conceive of time stopped, it requires the cessation of the flow of energy. Everything "freezes" as it is without change. All effects of time are seen as effects of the flow of energy (aging, etc.). All sentient physical beings experience and make their observations about the physical universe by the flow of energy within the brain, so the concept of the flow of time is inseperable from the thought process. The underlying physical law that is labeled the 2nd law of thermodynamics being that energy is transferred from matter with greater energy to matter with lesser energy is the same law that makes time flow in only one direction for all matter and beings made of matter.