Buddhism’s basic philosophical tenet, the Four Noble Truths, has its roots in the Buddha's tryst with realities of life during the course of his married life. The Four Noble Signs held the key to the four noble truths.
Although having every possible luxuries and comforts with a loving and dutiful wife and a beautiful son, Siddhartha's earlier experiences with life's deepest questions were slowly maturing and taking shape into some solid base of the realities of life. Siddhartha's early twenties were therefore disturbing indeed in spite of all the affluence showered upon him.
The first sign
A tradition says that before Siddhartha ventured out of the palace, his father would arrange the course in such a way that nothing related to suffering stood in the path of the young prince. However in of such trips, even after a meticulous arrangement by his father, Siddhartha is said to have come across the first of the noble signs – the sight of a an old man with graying hair, withered and wrinkled face and clothed with only a few rags. Turning to Channa, his favorite attendant, he asked him whether it was really a man for he had never seen one such in his earlier outings. Channa answered to his master that the man, though he was born like any other human, has aged in time and that every man goes through this stage before one finally dies. This was enough to sadden Siddhartha with a fact that even beauty ages in time.
The next two
In his trip after the first sighting of a mortal life, Siddhartha, accompanied by Channa set out by foot dressed in ordinary clothes so that his people would not recognize him.
While they were passing along one of the streets, they heard a cry of someone calling out for help. When they turned around to see what the matter was, Siddhartha was astounded to the see a man lying on the ground and twisting his body in the dust. Ugly looking purple blotches covered his face and his body; his eyes were rolling, and every time he tried to get up and walk, he fell helplessly down again. Channa cautioned his master not to go near the man as he might get infected by the peculiar fever that is plaguing his body. He again realizes the futility and the weakness of the human flesh.
Then they came across a little crowd of people in the street. They were weeping, and behind them came four men carrying a listless lean-looking man on a board. After a while, to his astonishment, Siddhartha saw the listless body piled with wood and then finally set on fire. Siddharta was dumb struck when Channa says that since every human is mortal, death will come to everyone anytime.
The Last Noble Sign
On another occasion, as Siddhartha was driving towards the Royal Gardens, he met a man dressed in flowing orange-colored robes. After observing the monk closely, and experiencing an inward pleasure of the calm, dignified man, he asked Channa about the person. Channa replied that the monk was one among the people who had "renounced the world" in order to seek an answer to the sufferings and sorrows of the world. After hearing this, the Prince was elated, spent the day happily in the Gardens with the thought that he have finally found a way to seek the answer to the world's suffering by renouncing the world.
These were the four noble signs which drastically affected Siddhartha's future outlook towards life. And these truths were the base upon which the four noble truths emerged as the basic tenet of Buddhism.