The core teachings of Buddhism are, “all things are impermanent”, “all things are nonself”, and “nirvana is tranquility.”
Modern science has proven that the sun, which seems to shine in the sky without changing, is actually changing every moment. We think that there is no change between ourselves of yesterday and ourselves of today, but the fact is that the cells of the human body are constantly dying and being born, so that all the cells of the body are replaced every seven years. Each cell of our body is changing continuously, though we are not aware of it. Everyone knows from experience how suffering, sorrow, joy or pleasure can change in an instant. It would be an error to think that things are consistent in this transient and unreliable existence. The Universal Law that "All things are transient", is the teaching by which we become aware of the changing nature of all things and therefore are not be surprised at or shaken by changes in phenomena or circumstances.
All things in this world, without exception, are related to one another. There is nothing that leads an isolated existence that is wholly separated from other things. When we consider that even tiny insects, birds flying high in the sky and pine trees growing on a distant hill were part of the same matter at the beginning of time on earth billions of years ago, we realize that these creatures are permeated by the same life energy that gives us life. The same applies to earth, stone, clouds and air. When we turn our attention to the present and consider our own existence, we know that if there were no clouds in the sky, we would have no rain; if there was no rain, no plants would grow; if there were no plants, we would have no food nor would the plants produce oxygen; and if there were no air, we could not live even for a few minutes. Without exception, we have some invisible relationship even with those things that seem externally to have no connection with us. Since most of the human body consists of water containing minerals, we live by the grace of salt, calcium, iron and copper. This fact demonstrates how things exist in connection with one another and are interdependent. It goes without saying that we have a much closer and stronger relationship to other human beings. We are inseparably bound with one another and we all exist permeated by the same life energy. In spite of this, opposition, dispute, struggle and killing cause each of us to be swayed by our ego, and to live selfishly for personal profit alone. This is the important reason why we must realize the truth of the teaching "All things are nonself." (Everything is interconnected)
This is an ideal state of mind and body that completely extinguishes all the sufferings of human life and obtains peace, balance and harmony. We can completely extinguish all the sufferings of human life and obtain peace and quietude when we destroy all illusions. How can we reach this state? The only way is to realize the two Universal Laws "All things are transient" and "All things are nonself." The reason we worry about various kinds of suffering is that we forget that all phenomena in this world are impermanent, that all things continuously change according to the Universal Law of “cause and effect.” We are deluded by phenomena and influenced by considerations of immediate gain or loss. If we study the way to Buddhahood and by practicing it realize the Truth of the transient of all things, we become able to attain a state of peace and quietude in which we can never be swayed by shifting circumstances. This is the state of "Nirvana is tranquility." We sometimes feel troubled by setbacks in business or personal conflicts. This is because we lack harmony with others and the world around us. Why are we not in harmony? It is because we have forgotten, or we do not realize the Truth that "All things are nonself" (Nothing has a separate self). We can attain harmony with others spontaneously when we remember the Truth that all things and all people are permeated by one great life energy, that all things are invisibly interconnected and when we make the best use of this interconnection by abandoning the idea of ego (separateness), it enhances the interconnection to benefit both ourselves and others. When in harmony with others, we can give up excess and deficiency, struggle and friction, and can maintain peaceful minds. This is the state expressed in the Universal Law "Nirvana is tranquility." (Transcending illusion will create tranquility)
There is a Fourth Law: All will be suffering, if we do not remember that “All Things are Transient” and “All Things are Nonself.” If the Truth is forgotten, suffering is the outcome. We can remember this at any time.