Thomas Edison had conducted over nine thousand experiments trying to device a new storage battery but not a single one succeeded. One of his friends, Walter Mallory, overcome with sympathy said, "Isn't it a shame that with the tremendous amount of work you have done you haven't been able to get any results?" Edison replied with a smile, "Results! Why, man, I have gotten a lot of results! I know several thousand things that won't work."
This little story tells us so much about the approach of assertion by way of negation, which the renowned eighth century CE philosopher Shankara propagated. The Sanskrit term for this is "neti neti" (na iti = not this) which essentially means "not this, not that."
When we define an object or an idea, we are basically limiting it. And the process of limiting something could be either positive ("this is what it is") or negative ("these are what it is not"). When the object or idea to be defined is straightforward and its characteristics are describable, then the positive approach is easier. But if the characteristics of what we are out to define are indescribable, then we resort to negating everything that it is not. This is also the import of the well-known Sherlock Holmes line: "Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth."
Shankara is not alone in this approach of rejection and assertion. The Upanishads often speak of brahman, the Supreme spirit, as beyond form and beyond thought. Lao-Tzu says, "The Tao that can be named is not the real Tao" (Tao-Te-Ching, Chapter 1). Buddha famously refused to answer the fourteen questions (about space, time, creation, identity, and life after death) for the simple reason that they could not be answered in words; his silence is a form of negation. The European idea of via negativa (negative way) is a similar approach of using negation, but in an attempt to define God. A parallel in the Judeo-Christian tradition is, for example, the refusal of God to reveal his name to Moses (Exodus 3:13-14) or St. Paul's reference to the "Unknown God" (Acts 17:23). In the case of Islam, most schools believe the Qur'an to be completely true and thus deem the attributes of Allah (Qur'an 59:53-54, for instance) to be literal. However in some schools of Shi'a Islam and Sufism, they approach God using ta'til (negation) - a popular example being Bulleh Shah's poem "Bulleh ki jaana main kaun" (Bulleh! Who knows who am I?)
One of the most famous compositions of Shankara on this topic is his Nirvana Shatkam, the six-verse poem to salvation:
[The five breaths are: digestive breath, excretory breath, upward breath that creates sound, balancing breath, and diffused breath that maintains body shape.
The seven sources of the body are: plasma, blood, muscle, fat, bone, marrow, and semen/ovum.
The five layers are: realm of matter, realm of vitality, realm of mind, realm of wisdom, and realm of bliss.]
We find an interesting counterpoint to the theory of negation in the writings of the fifth century CE grammarian, Bhartrhari. In his seminal work on grammar, Vakyapadiya, he discusses the paradox of unnameability - he says that by the very act of calling something unnameable gives it a name. It becomes signifiable precisely by calling it unsignifiable, for that has become its signifier.
While referring to the method of negation adopted in Brhadaranyaka Upanishad 2.3.6, Sureshvara (a primary disciple of Shankara) says that the objective of calling brahman as "not this, not that" is not simply to negate but actually to establish identity. We must keep this in mind while applying the approach of negation because it can easily lead us astray from our primary purpose - to define that which cannot be defined in words and to know that which cannot be grasped by thought.
Reference: Derrida and Negative Theology. Eds. Coward, Harold and Foshay, Tony. New York: SUNY Press, 1992.