The three prongs of the trident stand for the three thought textures (gunas), namely Sattva (pure and noble), Rajas (passionate and agitated) and Tamas (dull and inactive). Combinations of these three thought-textures in various permutations determine the different individual personalities. The noble, the ignoble or the indifferent traits in the human character are determined by the varying proportion in which these three constituents are mixed. When the thought textures are eliminated, one loses one's individuality and merges with the all-pervading Reality. The laying down of the trident indicates the transcendence of the gunas and the consequent merger of the individual with the Reality.
Lord Shiva is also known as the God of Austerity(tapas). He is often pictured clad in a deer skin, His body besmeared with holy ashes, and His long hair matted - all symbols of supreme renunciation. When, as Nataraja, He dances in triumphant frenzy His dance of realisation, the tresses loosen, suggesting that the state of realisation transcends austerities. Austerities are only a means - a path to the goal of God-realisation. When one has reached the goal, no need remains to adhere to the means.
Lord Shiva is represented as carrying the Ganga(Ganges) in his locks. The Ganges stands for spiritual perfection. According to a mythological story, the origin of the Ganges is Akasa-Ganga "Ganges of space", which was located in the heavens. King Bhagiratha prayed to the Akasa Ganga to descend to the earth and bless humankind. The prayer was granted.
So he implored Lord Shiva to receive her torrential flow with His strength, for none else could bear the force of her descent. Shiva, accordingly, received the water in His locks and disappeared into the jungle for meditation. The king once again prayed to Shiva to release the waters of the holy Ganges for the benefit of the earth. The story goes that the flow of the present Ganges is made up of water trickling from Shiva's locks.
Spiritual perfection is beyond intellectual comprehension, which is indicated by the Akasa Ganga being located in the heavens. In order to obtain it, one must first develop self-control, which Shiva proverbially stands for. After intellectually grasping the objective knowledge of the Truth, one has to retire to a quiet place for contemplation upon the knowledge so gained in order to bring about a subjective owning up of the Truth.
Thereafter, spiritual Knowledge can be communicated to others only in gradual doses, since others power of understanding the abstract knowledge is limited. This idea is indicated by the fact that the Ganges water descends from Lord Shiva's locks in a trickle. A dip in the sacred Ganges, therefore, is considered sacred, for it symbolises that the seeker has cleansed the ego and become one with the Supreme Reality.
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Swami Chinmayananda talks about the symbolism of Lord Shiva in depth. This article deals with the symbols of Shiva. It is an interesting read about Shiva in Hinduism, one among the important Hindu Gods.