In the evolution of man's knowledge, his mind experiences various states. What is the experience of a child cannot be the experience of an adult, and the experience of an adult will be different from that of a child. Tantra is a process of evolution of consciousness and Vedanta is an experience. Therefore, the relationship between the sadhana and ultimate realisation is the relationship between tantra and Vedanta.
In tantra there is the postulation of duality, whereas in Vedanta there is no duality, there is only one absolute. The ultimate experience when the mundane consciousness is annihilated is the experience of oneness or of non-duality. This experience is also known as adwaita, and adwaita philosophy is known as pure monism.
Now, if you analyse the meaning of the word Vedanta' it comprises two ideas - veda and anta. Anta means 'culmination' and veda means 'the complete process of knowledge and awareness'. When you are studying a particular subject, the whole effort that you make to understand the subject is known as veda. In the ancient Hindu culture, veda represents the higher revealed knowledge. Just as it is believed that the Bible, the Koran and the Zend Avesta were revealed, in the same way, it should be known that a process of revelation takes place in every enlightened person's life. Therefore, Vedanta means 'culmination of a process of knowledge'.
While you are passing through a spiritual process in life, you come across various psychic, occult or spiritual experiences. Some aspirants hear sounds, see the light and have fantastic experiences of the awakening of kundalini, but all these are transitory experiences, they are never permanent. There is a moment however, when the ultimate experience comes. This experience is known as the vedantic experience.
The Upanishads are the background or the basis and foundation of vedantic philosophy. If Vedanta is to be explained in a simple way, it is that One is ultimate and the multiplicity is the expansion of One. However, tantra does not talk of the ultimate experience, but of the process of enlightenment. In tantra there is duality; there are two realities - Shiva and Shakti. As in hatha yoga you have ida and pingala nadis representing chitta and prana, in the same way, on the cosmic level, you have Shiva representing the total matter and Shakti representing the total energy.
Now, the union that takes place between Shiva and Shakti is considered to be the ultimate experience. This means all experiences we have in spiritual life are the result of a union or combination of the two forces within as. Philosophically, therefore, we can say that every experience in spiritual life is a product of duality. You may see light or you may hear voices; you may have any kind of spiritual experience, but it is a product of duality, not of non-duality.
As such, all experiences which we have during life, either on the sensorial or the spiritual planes, are not absolute experiences. The experience of a siddha, a miracle man, a clairvoyant, telepath, prophet or anyone, is an experience produced by an interaction of two forces. The basis of all experiences is human consciousness, and these experiences are aided by the sensorial channels. When the senses and the mind interact with each other, they produce a true or a false experience. If your mind is purified and your senses are under control, you will have more tangible experiences, but not the absolute experience. If your senses are unruly and your mind is full of dross, then the experiences which you have will be faulty.
Fundamentally, tantra and Vedanta can be viewed as two different traditions, but ultimately, when they are to be understood in the context of human experience, they have to be seen as systematically linked with each other. For example, in mathematics, first distances, measurements and weights are taught. Our children learn about them in primary school, but when they are in high school, they are taught that all the distances and weights are false. If they are false, then why did they have to learn them? Because there are two forms of knowledge. One is relative and the other is absolute.
In spiritual life, as long as you are acting through the mind and senses, you require relative knowledge, but as soon as you transcend these tools of knowledge, the absolute knowledge dawns. Even though the truth is absolute, you cannot deny the necessity of the relative processes of knowledge. In order to climb to the rooftop terrace you need the staircase, and in the same way, to attain an absolute experience, you have to go through the relative processes of experience. This is precisely the link between dualistic tantra and monistic Vedanta.
Most scholars have difficulty in understanding this. They think if there is duality, it should always be duality and if there is one, it should always be only one. This is a mistake. When we say that absolute is one, we conceive of that one in the same way as we think of one rupee, one pound or one dollar. But one is not a limited unit, it is the totality of everything; it is a synonymous term used for absolute. In Sanskrit it is known as poornam, and there is a famous mantra which you might have heard: This is poornam; that is poornam. From the poornam the poornam emanates. When the poornam is deducted from the poornam, what remains is poornam.'
Now, to put this into mathematics: infinity plus infinity is equal to infinity. Infinity minus infinity is equal to infinity. Well, if you say zero plus zero, it can't be one, and zero minus zero is also not one. This is precisely the relationship between dualistic tantra and monistic Vedanta. When Einstein was asked to define relativity and absolute, he gave his famous equation. People said, 'No, we want to understand it in a different way.' So he said, 'Both are phases of human experience; one depends on external factors and the other on the reality.'